Are you puzzled about the types of motor oil available? Wondering if you're choosing the right one for your car? Here's a crash course, so to speak:
Multi-viscosity or straight-weight.
Multi-viscosity oils are those denoted by two numbers with a 'W-' in between (for example, 10W-30 or 20W-50). The 'W' originally stood for winter, as oils were developed to compensate for the wear due to the vast temperature ranges then. They're almost exclusively used today rather than straight-weight (monograde) oils. Multi-viscosity oils use polymer additives to change the natural consistency of oil as it's heated, preventing it from thinning out as much as it otherwise would.
The only downside to this is that some of the additives used to improve viscosity at high temperatures can also leave sticky deposits. Oil companies have mostly eliminated such problems by adding detergents to oil, along with rust inhibitors and antioxidants. Some precise performance engines will still require straight-weight oil, but all newer vehicles are designed for multi-viscosity oil and require it, else your warranty will probably be void.
What the numbers mean.
The numbers in motor-oil nomenclature refer to the viscosity (thickness) of the oil. A higher number corresponds to thicker oil, while a lower number refers to thinner oil. The numbers themselves are supposed to correspond to a set of real, measurable qualities in the oil, one of which is the viscosity index. In multi-viscosity oils, the left number refers to cold behavior of the particular oil, while the right number refers to its hot (100 degrees Celsius) behavior. So, for instance, 5W-30 oil would flow well when cold like 5-weight oil, but protect at high temperatures like 30-weight oil.
Do oils vary much by brand? Yes. The 5W-30 oil from one company might equal 10W-40 oil from another company in viscosity, because there might be a difference in some of the other properties of the oil, like its flash point (at what temperature it ignites). The numbers in multi-viscosity oil also don't tell anything about how viscous the oil is at normal engine operating temperatures, as opposed to extremes. The best advice here is to choose a familiar brand, and experiment with other major brands if you think it's too thin or too thick. Never mix brands or different weights. Wait until an oil change.
What about synthetics?
Synthetics are better in every way, but they are much more expensive. They offer better high-temperature resistance and better low-temperature flow, and they leave nearly no deposits. Prices for synthetics are coming down, though, and there are also blends that combine synthetics with traditional mineral oils. Because synthetics flow and penetrate much better than regular oils, a change to synthetic oil will sometimes reveal leaks you didn't know existed.
The American Petroleum Institute (API) assigns letter ratings to motor oil. The category ratings correspond to industry standards for various qualities like viscosity, thermal protection, and preventing deposits and sludge. Every few years, the currently awarded rating is changed. Currently, SL (Spark ignition, Category L) is the top rating. SJ was introduced in 2001 and SH in 1996.
As your car ages and becomes a high-mileage vehicle, many experts recommend that you change to thicker oil than is normally used. Over time, gaps between parts in the engine become larger, enabling less oil to reach critical parts. Ask a mechanic familiar with your model vehicle or ask the dealership if you should change the type of oil for your vehicle beyond 100,000 miles.
The underlying message is that all oil is not the same. Making sure you use the correct viscosity rating is just the start. The quality and characteristics of oils can vary greatly by brand, too.