I'm very confused about motor oil For instance there are...

Dear Car Talk

Dear Car Talk | Aug 01, 1994

Dear Tom and Ray:

I'm very confused about motor oil. For instance, there are two-letter ratings on the sides of the container. Some say "SG," and some say "SH." What do they mean? Thanks.

TOM: The ratings are performance specifications for the oil. Basically, the higher the second letter, Lewis, the better the oil.

RAY: SH is currently the top of the line, but you may still see both SH and SG on store shelves. When a new designation appears, it just means that the standards have been raised, and the new oil (in this case, SH) meets those higher standards.

TOM: The reason the standards keep going up is not due to some noble pursuit of divine lubrication. It's because engine manufacturers are trying to meet increasingly stricter government regulations for fuel economy and emissions. And improving engine lubrication is one way to help meet those standards.

RAY: For instance, one way to improve fuel economy is by using a less viscous (thinner) oil, like 5W30. When oil is thinner, internal engine parts move more easily, and the engine uses less fuel. But one problem with "thinner" oil is it tends to evaporate more easily than thicker oil....which means you need to add oil more often. So the SH oil meets higher standards for "volatility," which means it doesn't "burn up" as quickly.

TOM: The SH oils also have better "deposit control," which means they hold more engine contaminants in suspension and prevent engine deposits. They also have better "shear stability," which means they maintain their viscosity over time. And they meet new standards for "filterability," which means the oil can be filtered properly even when moisture from frequent, short-trip driving condenses and makes the oil more "watery."

RAY: So which oil should you use? Well, it never hurts to use better oil than you need. So, for instance, if you bought a 1992 car, which calls for "SG" rated oil, then it's fine to use "SH."

TOM: And even if you own an old heap like my brother's 1963 Dodge Dart, you can still use "SH," even though it originally called for ???+ (they were still using the Greek alphabet when his car was made).

Get the Car Talk Newsletter