Most of us have only a vague understanding of viscosity. We tend to choose an oil with a viscosity that we believe is correct for our particular engine, but would another viscosity improve or reduce the life of the engine? Can we freely pick and choose a viscosity outside a manufacturer's recommendations?
Technically, viscosity is defined as resistance to flow. Commonly though, we think of it as an oil's thickness. To be more specific, it is the thickness of an oil at a given temperature. The plot thickens (ha!).
The viscosity of an oil could be reported at any temperature, but to standardize things, most laboratories report either a low temp (100F or 40C) or a high temp (210F or 100C) and stick with either SUS or cSt. The standardized temperature reading allows us to compare apples to apples for judging the thickness of the oil. At Blackstone, we report the viscosity at 210F SUS, which is about the operating temperature of your engine when it's warmed up and running.
An apple is an apple, no matter what language you use to describe it. In the same respect, there are many ways to describe viscosity: SAE Engine, SUS (Seybolt Universal Seconds), cSt (Centistokes), ISO grade, etc. We use SUS. No matter what you call it, the number given simply defines the thickness of the oil at the standard high temperature.
Straight Weight vs. Multi-Grade
The difference between multi-grade and straight-weight oil is simply the addition of a viscosity improving (VI) additive. The most common grade of automotive oil in use today is the 5W/30, which is a mineral oil refined with VI additives that leave it reading as an SAE 5W viscosity when cold, yet an SAE 30W when hot (210F). The advantage to the multi-weight is that when starting the engine, the multi-viscosity oil (with its thickness of an SAE 5W when cold), allows the engine to spin over more easily.
The most common diesel use oil is 15W/40. It is an SAE 15W oil with a VI additive that leaves it the thickness of an SAE 40 weight at operating temperature. What makes an oil a diesel-use oil (rather than automotive-use) is the level of additives used. Diesels require heavier levels of dispersant and anti-wear additives. These heavier additive levels are objectionable for automotive engines since they may interfere with the emission controls mandated by the EPA.
Which Viscosity to Use?
Changes in Viscosity
Other changes to viscosity can result from contamination of the oil. Moisture and fuel can both cause the viscosity to increase or decrease, depending on the contaminant and how long it has been present in the oil. Antifreeze often increases an oil's viscosity. Exposure to excessive heat (leaving the oil in use too long, engine overheating) can also increase viscosity. When your oil's viscosity comes back as either lower or higher than the "Should Be" range, something is causing it. If the high/low viscosity is hurting wear, the key is to find out what it is and repair your engine or adjust your driving habits accordingly, to correct the viscosity and optimize your engine's efficiency.
If you decide to use a different viscosity oil than what the manufacturer recommends, you might want to use oil analysis while you are experimenting. Your wear data doesn't lie. People selling oils and additives may be sincere, but they don't have to live with the results. They simply smile a lot on the way to the bank.