Dear Tom and Ray:

I change my own oil and filter on my car. A few years back, I ran into a real deal on a few cases of 5W-30 oil rated at SF. Recently, I traded in my car and the new car requires 5W-30 rated SG. I have 15 quarts of SF left. What is the difference between SF and SG? Could I mix two quarts of SF with two quarts of SG and use up the SF that way? Would it hurt to add a quart of SF to the SG when I'm down a quart between changes?
Hugh

RAY: Hugh, we can tell you're a card carrying cheapskate. Don't you know the famous Click and Clack axiom? "It's the Stingy Person who Spends the Most." You can probably get by with SF, but why take a chance on damaging your brand new engine just to save a few bucks on oil?

TOM: SF and SG are ratings for engine oils. The second letter measures, among other things, the oil's ability to lubricate the engine, neutralize acids, and hold contaminants in suspension. The later the second letter is in the alphabet, the better the oil. SG is currently the highest rated oil you can buy.

RAY: They're hesitant to improve the oil any more, because then they'll have to go to the SH rating, and they're fully aware of the implication of that.

TOM: Don't chance it, Hugh. Put a bow around that case of SF and give it to a neighbor with a lawnmower.