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 (pun intended).

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 Fahrenheit or Celsius.
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.

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

Engine oils can be either a straight weight or a multi-grade viscosity. Originally, all oils were straight weights. Relatively few straight weights are manufactured today since most gas- or diesel-engine manufacturers recommend multi-grades.
At operating temperature, a straight weight performs just as well as a multi-viscosity oil, and there is nothing wrong with using a straight weight. It's just a simpler form of oil. Some diesel fleets still use straight weights, as do about half the piston aircraft operators.

The difference between multi-grades and straight-weight oils 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 to the SAE 5 weight viscosity range containing the usual cleaning and anti-wear additives, then blended with a VI additive that should leave it reading in the SAE 30 weight range when at the higher (210F) temperature.
The advantage to the multi-weight is, when starting the engine, the multi-viscosity oil has the thickness of an SAE 5 weight, which allows the engine to spin over more easily.

The most common diesel use oil is 15W/40. It is an SAE 15 weight 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?

Engine owners often stray from manufacturers' recommendations regarding viscosity of oils. The engine builders dyno-test their engines using a specific viscosity oil, so when you use the viscosity they recommend, you are working with a known result.
Going to another viscosity is an experiment, but it's usually a harmless one. For the sake of efficiency you want to run the lightest grade oil in your engine possible, within limits. We are seeing that trend for newer engines, for which the recommended grade is getting progressively lighter.

The common 10W/30 has become a 5W/30, and some manufacturers even recommend 5W/20 oil. On the other hand, we can't see (in oil analysis) where it hurts anything to run heavier 10W/30s or even 10W/40s in modern automotive engines. The heavier oils provide more bearing film, and that's important at the lower end.

If your oil is too light, the bearing metals can increase. If the oil is too heavy, the upper end metals can increase. The trick is to find the right viscosity for your particular engine, which is why we suggest following the manufacturer's recommendation.

Changes in Viscosity

Adding anything foreign to your oil can change its viscosity. Some types of after-market oil additives cause a quite high viscosity at operating temperature.
While an additive might improve bearing wear, it can often cause poorer upper-end wear. We don't recommend any type of after-market additives.

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. 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.

The Silicon Bugaboo

Stating that the element silicon – when it appears in the spectrometry of oil – has one source and one source only: abrasive dirt.

That was wrong.
Many people believe silicon only comes from dirt. That point of view is so firmly set that laboratories using only computer-generated comments often state that high silicon is from abrasive dirt and do not take into account the many other sources of silicon (additives, sealers, etc.).
Neglecting to take into account the other potential sources of silicon in an oil is as much an error as the gentleman expounding on silicon in our lobby.

In oil analysis spectrometry we look at elements rather than molecules. When we read any element, we can only assume the molecular form from which they came.
Here are several items that can cause high silicon readings in an oil sample. Some forms will harm an engine, and others will not.

Silicone-based gasket sealers (red, blue, or clear) used in engine assembly and repair, show up as high silicon in an oil sample. Any oil sample taken from a new, rebuilt, or repaired engine typically reads high in silicon.
Oils from some types of new engines may read as high as 400 ppm silicon. You can ignore the high silicon in these cases since it is harmless and will wash away by the third or fourth oil in the engine.

Aftermarket oil additives often use compounds that will leave silicon reading high in an oil sample. There is a compound for instance, used in anti-foam additives, that can run silicon up to as high as 70 ppm. This form of silicon will not affect engine wear.

Silicon often reads high in an oil when coolant seeps into the oil system. It comes from silicon (rust) inhibitors used in the anti-freeze. While the inhibitors aren't necessarily harmful, the anti-freeze certainly is, and you will want to seek repairs before the engine is seriously damaged.

There are certain types of gaskets used at pan and valve cover areas that are made from silicone-based materials. If these are installed on your engine, you can expect the silicon reading from your oil sample to go up to 15-20 ppm and continue reading high for as long as the gaskets are in place.

Analysts consider all these factors when making recommendations about the condition of your engine. The more information you give us regarding any additives used, rebuilds the engine has been through, or any other special considerations we should take into account, will help us determine the true health of your engine. We can then make appropriate recommendations to help you prolong its life. And in the end, that's what it's all about.

The insolubles test measures the total insoluble materials in an oil sample, that is, all solid or liquid materials that are not soluble (won't mix) in oil. Virgin oil usually doesn't have any insoluble materials in it. When it occasionally does, the most we normally find is a trace level. The insolubles in virgin oil are from the normal oxidation process of the oil, which leaves free carbon in suspension when oxygen forms with hydrogen (oils are hydrocarbons).

The insolubles test is a centrifuge method. A measured volume of oil is mixed with a heated solvent, agitated, and spun at high speed. Insoluble materials collect at the bottom of a tapered glass tube and can then be quantified. The insolubles test is a fair measure of how fast the oil is oxidizing and receiving contaminates, and how effectively the system's oil filtration is functioning.

Industrial oils normally contain very low insolubles due to the few and relatively mild heat cycles the oil experiences (heat cycles accelerate the oil's normal tendency to oxidize). Further, oil filtration on industrial machines may filter particles as small as 210 microns, keeping the oil pristine for a very long time, often years.

Automotive and aircraft oils however, suffer the most difficult environmental problems of all types of oils we analyze. They regularly receive blow-by products from the combustion process. They suffer extreme heat cycles. Any contaminate in an oil will accelerate the oxidation process, causing insoluble materials to increase. Engine oils need to be changed regularly due to all of the above.

Excessive insolubles can form in an engine oil if the oil: is running hot, is receiving more than a normal amount of contamination, is suffering more (or more severe) heat cycles than is normal, is being run longer than a typical use cycle, or, on the other side of the coin, if oil filtration is marginal or relatively ineffective.

If we found no contamination in your oil and your use intervals are normal, we often mention a problem at oil filtration as a possible cause of higher insolubles. Insolubles may be forming because your oil use interval is too long for the condition of the engine. Your oil filter may be inferior. It is possible the oil filter bypass valve has relived if the filter is becoming restricted. The filter system bypass may also open upon unusually cold starts when the oil is too thick to pass through the filter media. Once the bypass relieves, the filter is effectively out of the system.

The insolubles test is a fair measure of several possible problems in your engine. One high reading needn't be a cause of concern. Several high readings in a row merit investigation of what the problem may be.

Here is an example of having manual trannie fluid analyzed by a testlab

MM6 Trannie fluid Analyzed