What’s the Best Oil Change Interval?
Here’s an interesting question we received recently from Pete, one of our long-time customers:
“Needed to ask you about AMSoil OE synthetic oil. The change interval that is suggested for that oil is whatever the vehicle manufacturer has specified for that particular vehicle. But what have you seen from the TBNs that you have run on that oil? It doesn’t make sense that in my old 01 Maxima it would only be good 3000 miles but in a brand new vehicle it could be good for 10K.”
Pete has a point. Why would the same oil wear out faster just because the manufacturer recommends a shorter oil change interval? If the oil can hold up for 10,000 miles or more in some engines, shouldn’t it be able to do so in any type of engine?
Of course, there are some reasonable explanations that Amsoil (or any oil manufacturer) might give for this. The industry is generally moving towards longer oil change recommendations because modern engines are built to more exacting standards than they were even ten years ago, allowing for improved efficiency and less damage to the oil.
Plus, you might have to adjust the oil change interval in the same exact engine based on whether you’re seeing “severe” duty or not, so it’s probably reasonable to think that some types of engines would just treat the oil a little more harshly, and require a shorter oil run, right?
The cynical side of me, though, says that the real answer to this question probably has more to do with the legal department than the oil’s engineers. Regardless of how good you think your oil is, if you start telling customers they can ignore the original engine manufacturer’s recommendations, you’re probably opening yourself up to some legal headaches that the head office just doesn’t want to deal with.
But true as that may be, it’s not a very good answer for Pete, or the rest of our customers who are just looking for the best advice on how to treat their vehicles. So setting aside specific recommendations for a moment, let’s get to the nut of Pete’s question… Does the life expectancy of the oil change based on what engine it’s used in?
The tale of the TBN
There are several factors that we use to determine if your oil can be run longer, but Pete asked specifically about the TBN, so let’s focus on that for now.
For those who don’t know, the TBN (Total Base Number) measures the amount of active additive remaining in the oil. A typical gasoline-engine oil might have a starting TBN between 6.0 and 8.0, while diesel-use engine oils tend to have higher TBN’s of 11.0 or 12.0, since they have to deal with dirtier, more acidic conditions.
But regardless of where a TBN starts, they all end up in the same place—0.0—if the oil is run too long. Once the TBN is down to zero, it means that the oil is no longer able to neutralize acids produced by the engine. As a general rule of thumb, we usually say that once a TBN gets lower than about 2.0, it would probably be a good idea not to run the oil much longer, to avoid running out of those active acid-neutralizing agents.
To answer Pete’s question about how the TBN of Amsoil OE holds up in different engines, we searched our database to find the recent TBN results we’ve seen from that type of oil. Since the TBN is an optional test, we don’t run it on every sample we see, but customers who use Amsoil tend to be pretty interested in seeing how long they can run their oil, so we have a pretty good representation for that oil type. We plotted nearly 50 samples’ TBN results against the mileage on the oil, and came up with this graph:
The banana-shaped line we’ve drawn approximates the “average” TBN for this type of oil over a given mileage. You can see that the TBN tends to drop quickly at first, but the longer the oil is run, the slower the TBN drops. Of the samples we tested, none of them had a TBN less than 1.5, so even on very long oil change intervals, this Amsoil OE oil tends to retain plenty of active additive.
Also note that the actual test results (the dots) can stray pretty far from that line on either side, so even though the TBN readings tend to follow a particular pattern, there can be a pretty wide deviation in individual test results. Just at a glance, you can easily see that the sample with the highest TBN reading didn’t have the lowest mileage on the oil, nor did the sample with the longest oil run (a whopping 18,500 miles) have the lowest TBN reading. In fact, the lowest TBN came after a fairly middle-of-the-road 8,569-mile oil change interval.
And before we get too hung up on looking at just Amsoil OE, we ran the same analysis for one of the most common oil types we’ve tested, Mobil 1 5W/30, resulting in the following chart:
The graph for Mobil 1 5W/30 covers nearly 5,000 samples with TBNs, and the scale is a little different than the Amsoil OE chart, but you can see that banana-shaped curve that we’ve drawn, approximating the average TBN for a given mileage, is exactly the same as in the Amsoil OE chart. Once again, the highest TBN was not the shortest oil run, and the lowest TBN was not the longest oil run. So even though we have many more TBN data points for Mobil 1 as we do Amsoil OE, the overall trends for TBNs are similar, and would be with just about any type of oil you could name.
What affects the TBN?
So what other factors might be affecting the TBN? To find out, we ranked all the samples according to both mileage and TBN reading, and came up with the best and the worst of the bunch.
One factor that definitely stood out was make-up oil. If you add some fresh oil in between oil changes to top up your oil level, you’re infusing the oil with more active additives, and diluting wear metals and contaminants at the same time. That’s why we often say that you shouldn’t be too upset about adding a quart or two of oil over the course of your regular oil interval (assuming you don’t have a noticeable leak, of course), since that fresh oil might buy you a few thousand extra miles before you have to do a full oil change.
In this case, all of the samples that noted adding a quart of oil or more ranked in the top half of the results, and the Amsoil samples with the most oil added (2.5 and 3 quarts) ranked at numbers 2 and 7, respectively. On the other hand, the overall best-ranked sample, with a TBN of 4.0 after 10,000 miles, didn’t add any oil in that time, according to their oil slip, so make-up oil alone is not the only relevant factor.
Pete’s original question was about manufacturer’s recommended oil change intervals. We don’t have a list of the recommended OCI for every engine we’ve tested, so we’ll have to settle for looking at some other factors, like the age and size of the engine.
For both the Amsoil and the Mobil 1 samples, the age of the engine didn’t seem to make much of a difference. We had engines from the late 90’s and early 2000’s in the top ten percent on both charts, mixed right in with new engines from the last few model years. The total engine mileage was also mixed, with higher-mileage engines ranked right alongside brand new engines in their first few oil changes.
Engine size is one factor that I thought would end up playing a pretty big role, but I wasn’t sure which way it would go. On the one hand, larger engines tend to work harder, so I wondered if the larger 6-cylinder engines and big V8s might burn through the active additive more quickly than smaller engines. On the other hand, 4-cylinder engines also tend to have smaller oil sumps, meaning less total oil volume in the engine, so maybe their active additives get used up sooner.
Mixed results
Turns out there results were pretty mixed as well… there was a bit of a trend for smaller engines to hold a higher TBN for longer oil runs, but there were plenty of larger engines near the top ranks of both lists, and vice versa. Seems like the extra oil in the sump of the larger engines pretty much balances out the extra work they have to do, resulting in a mostly even distribution of engine sizes across the rankings in both charts.
So what does all this tell us? Well, at least as far as the TBNs go, it doesn’t look like the type of engine has much of an influence on how long the active additive lasts in the oil. Engines of the exact same type (and in some cases, even the exact same engines) were ranked both high and low in our results, so it looks like individual driving habits and the behavior of each particular engine play a much larger role than engine sizes, model years, manufacturers, or any other criteria we could see.
When you get right down to it, though, the TBN is only one factor in determining whether or not it’s safe to run the oil longer. It’s a valuable tool, but we also have to look at other factors, like wear metals, insolubles, viscosity, and contaminants, any of which could indicate that you shouldn’t run the oil any longer, even if the TBN is still good.
The bottom line is this: the lawyers have to tell you to follow the engine manufacturer’s recommendations, since they have no idea what’s going on with your particular vehicle. And really, the original engine manufacturer doesn’t have much better of an idea—they know how their engines should wear, and base their recommendations on what should work best for most drivers, but they don’t have any idea about your particular driving habits or maintenance routines.
When you get your oil analyzed at Blackstone, we’re looking at the specific conditions for your specific engine, which is why we can tell you if it’s safe to add an extra 2,000 or 3,000 miles on your next fill, regardless of your current OCI. Just check “Yes” next to the question “Are you interested in extended oil use?” on the back of your next oil slip, and maybe you too will be free to explore the world of extended oil use!
If you have them, trends are the most helpful thing we look at in determining your engine’s health. It takes three samples to get a good trend going (though we can often tell if something is amiss earlier than that).
If we don’t know what kind of engine you have, we might end up comparing your numbers to the wrong set of averages, or just a generic engine file. We can still tell if something is way out of line, but the more subtle differences between your engine and averages are harder to see.
HyVis 4 Winter
additive in it, just not any that we read. The grade is not listed on the can, but the viscosity came back as a light 20W. With the lack of additive it’s not surprising that the TBN read 0.0. It’s tempting to do an experiment and run this apparent mineral oil for 10,000 miles in the dead of winter, just to see what would happen. If you know of any guinea pigs, send them our way!
the country, and I’ll always associate the blue, red, and white logo with long trips across the country in our green van with the velour bed and hanging beads. But Amoco did more than fill up gas tanks in the 70s–they also sold oil, and this “Long Distance” version is an SAE 10W/40. The additive package looks a lot like the Mobil Special oil we saw: heavy on phosphorus and zinc, lighter on calcium and magnesium (Figure 2). Just the right oil for a couple of bandana-wearing hippies traveling with two little kids from Indiana to Nova Scotia in 1976 in a green van with a sunset painted on the side. Ah, the ’70s.
The viscosity read like what we see today out of a standard 5W/30. Texaco Havoline Super Premium 10W/40, on the other hand, looks a lot like one of today’s diesel-use oils in additives (Figure 4), with a normal 10W/40 viscosity.
only did they not make multi-viscosity oils that long ago, but the can comes from zip code 90017 so it has to be from 1963 or later. According to the can, it’s 100% parrafinic oil with selected additives, which our spectrometer reveals to be your standard line-up of calcium, phosphorus, and zinc (Figure 6). Look at that viscosity though¾it’s higher than we see in today’s 20W/50s. 
Like modern oils, they stress the high quality of the oil with terms like “organo-metallic” that are meant impress those of us who aren’t in the oil business. I don’t know if I’d call this a masterpiece in oil work though: it looks like what we see out of ATFs these days as far as additives go (mostly phosphorus with a little zinc thrown in). Since the word “Racing” is stamped into the top of the can, the thick viscosity (like a 50W) makes sense (Figure 7).
I’ve always had a soft spot in my heart for Sinclair. The can has a picture of a dinosaur on it! This shit came from the ground, no doubt about it! Another 20-20W oil, the oil is light on advertising copy but heavy on additives. In fact, it looks a lot like recent versions of Shell’s Rotella 5W/40, except with a little less calcium. It’s much lighter than Shell’s 5W/40, though, with a viscosity reading like a 30W or a heavy 20W oil. Note the presence of lead (Figure 8). 
Modern Amsoil products tend to be heavy on additives and that was true back in the day as well (Figure 9). Just for fun, we compared this oil with a virgin sample of Amsoil 10W/40 that we ran in February 2012 and they look a lot alike. The only difference is in the older oil there’s less of everything: 500 ppm less calcium, and 100-200 ppm less phosphorus and zinc. They used magnesium in the older oil, while the TBN and viscosities were nearly identical.
over these two old cans. Originally, Penn’s Oil came from an oil field in Pennsylvania and was christened with a Liberty Bell logo to remind users of its Pennsylvania roots. This can of ATF clearly has an earlier generation of logo on it, and Google informs me that Dexron II was introduced in 1972. This may very well have been the transmission oil that kept our green van chugging through the ’70s. There are a lot of different ATFs in stores today, though generally they have about the same additive configurations. This one is a little different in that it has more boron, magnesium, and zinc than most modern ATFs. The viscosity is right where we’d expect it to be though. Pennzoil’s 10W/40 oil can is flashy, a la the 
1980s. It’s “The Motor Oil With Z-7” and although they don’t specify what that is, they do specify that “You need no extra oil additive.” So that’s reassuring. It’s rated SF-SC-CC, so I’d place it at about 25 years old. Maybe the magic of Z-7 is copper: that’s something we saw a lot of back in the day, when Blackstone was founded. Interestingly, magnesium is the dominant additive in this one, followed by zinc, phosphorus, copper, and boron (Figure 10). The flashpoint was lower than what we see today from 10W/40s. 
a can like the others. This one was sold in a 5-quart metal jug. I particularly love this one, because the can not only says you’ll “Cut Your Oil Bills In Half,” but the first point of advertising on the side is a “Faster Getaway!” Now, they don’t actually say that this is the choice of oils for bank robbers, but I know if it was 1941 and my hungry Great Depression self was contemplating which oil to put in my Ford Special for bank-robbing time, this is the oil I’d pick. With practically no calcium or magnesium present, the oil’s TBN read 0.0, but it does provide would-be crooks with phosphorus, zinc, and barium as well as a 20W viscosity for the getaway (Figure 11). When you’re busy working a tommy gun, the last think you want to think about is whether or not you’ve made the right choice in oil.
Wolf’s Head SD 10W/40
Wolf’s Head ATF
white, and blue, so you can feel patriotic when you buy it (unless 
you’re in Canada. Then you can indulge in justified rage about Americans and how we think we’re the center of the world). Fox Head oil was made by the Tritex Petroleum company out of Brooklyn, NY, and my extensive research (aka first-page Google results) tells me the company still exists and is presently located in Tulsa, Oklahoma. The logo, a sly-looking fox, has nothing on today’s slick oil packages. And the oil itself also has nothing on today’s oils: the oil itself is nearly bereft of additives. Basically a mineral oil, it has a little magnesium, phosphorus, and zinc in it, and not a lot else (Figure 3). This is not necessarily a problem, however. As you’ll recall in the article when Ryan used 30W aircraft oil in his truck, wear went up a little but the engine didn’t fail or anything. Still, I won’t be putting it in my Outback anytime soon.
long they’ve been making this oil and the guy not only could not tell me, but he was unable to tell me who might know. Surely someone at that company has a historical file? If so, they’re not sharing that info with plebeians like us. He did mention that Rotella really made its name in the ’70s, though I’m guessing this can of SF, SE, SC oil was made in the late ’80s. He also said the “S” versions of Rotella were sold internationally, and indeed, this can came from our friendly neighbors to the north (*waves hi to Canada). Suffice it to say that the oil has changed very little over the years. Its main additives are the same as what we see today, but the interesting part of this oil is that it’s a 10W (Figure 4). We often see heavy-duty thin-grade additive packages in tractor-hydraulic fluids, which are used in systems like transmissions and hydraulic systems in off-highway equipment like bulldozers and backhoes. Note the TBN of this oil read higher than most of the others we’re talking about. That’s because of the high calcium level¾the TBN is based on the level of calcium sulfinate and/or magnesium sulfinate. When those compounds aren’t present, you get a low TBN.
Quaker State 30W & 10W
these oils are mostly the same; they’ll throw in a few slight differences in additives and call it good. These cans of Quaker State, however, were mostly pretty different. The Deluxe version looked a lot like their 30W oil (but more calcium–Figure 7). Quaker State Sterling HD 10W/40, on the other hand, went out on a limb with almost 800 ppm sodium, almost no magnesium, and then levels of calcium, phosphorus, and zinc that are comparable with today’s oils. Touted as “Energy Saving Motor Oil,” Quaker State was getting its game on in pushing this brand: it mentions “special friction modifying additives,” the longevity of the company (over 60 years when the can was made), and its suitability for those wishing to follow extended drain intervals. Heck, I’m sold, and I see this stuff all the time.
Mobil is no slouch in the marketing department, but they really outdid themselves with the can we tested, “Mobil Special.” The name alone tells you all you need to know about why to buy this oil. All oil companies like to mess with their additive packages, and Mobil, like the others, changes their oil up fairly frequently. That was the case back in the day too, because the additive package in this “Special” oil is different from what we typically see in today’s oil. Apparently Mobil was an early rider on the ZDDP train, because this oil is chock-full of both phosphorus and zinc. Calcium and magnesium are present too, but at lower levels (Figure 8). We also tested a sample of Mobil Artic oil. The Artic
can is clearly older than the other Mobil can¾the logo is older, and there’s no zip code listed with the address, so it’s pre-1963. A straight 20W, it’s labeled as HD oil, meeting “Car Builders’ Most Severe Service Tests.” While it’s “Artic” and not “Arctic,” we can’t help thinking this oil is meant for cold-weather operation. The can even looks like it’s ready for winter: all white, but with a little color on it so you don’t lose it in the snow when you’re out in the tundra changing your oil. This one definitely has an unusual additive package, relying heavily on barium (maybe it’s got a purpose after all!). Interestingly, less zinc is present than phosphorus (Figure 9). Nowadays it’s the other way around.
see out of modern 15W/40s¾a stout additive package and a relatively thick viscosity (Figure 10). In other words, even though this oil is several decades old, it would be fine to use in your F150 tomorrow. 
and needed some oil for my Passat. It just starting to clatter a little on start up and when I checked the oil, it was down two quarts. The clatter sounded something like “Sell Me” in German.
One thing led to another and before I knew it I had bought 28 cans of old oil and spent almost $1,000. Pretty soon these oils started rolling in and I experienced a little buyers remorse. Did I really need to buy all this? What was I going to do with the cans? Once you open a can of oil, it’s almost impossible to seal up properly. Would there be anything to even see in these samples? And, does oil go bad? We get this last question all the time, and my answer has always been no, but I was dealing with oils from the 1930s,1940s, and 1950s here–really old stuff. Maybe all the additive in there (if any was even used) would settle out and there wouldn’t be anything for us to read. Fortunately, I had bought some oil that would help answer that.
However, that really didn’t get down to answering the question: Is this oil still good to use? For that, I was going to have to run another test.
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Looking at the results, I’d say this oil is indeed deluxe. The viscosity is pretty strong for a 10W/40, and the additives would be suitable for diesel use. The oil does have a CC rating as well as an SE rating, and those put the date of this oil as being made sometime in the 1970s. The ATF has a standard additive package until you get down to barium. That’s not used much anymore. See figures 5 and 6 for the analyses.
Valvoline SAE 20W – API SB
