Dear Tom and Ray:
When purchasing oil, the oil has a quality level (API Service). There are many different categories/types, such as SG/CD, SF, SM, SA, SB, SC, SE, to list a few. Today when I purchased oil, all the available viscosities and brands were SM. I have a 1993 Ford van with a 4.9-liter 6-cylinder engine. The owner's manual recommends 10w-30 SG/CD, which I could not find. What do all these types mean? Do I need to be concerned?
RAY: No, you don't need to be concerned, Dave. The letter combinations are performance ratings from the American Petroleum Institute, or API. When your van was manufactured, the top-of-the-line motor oil was rated SG, so that's what Ford called for.
TOM: Well, since then, oils have continued to improve. They now last longer, lubricate better and hold more contaminants in suspension. And as oils have gotten better, they've been given higher designations. SH replaced SG in the top spot. Then came SI, SJ, SK and SL.
RAY: And now the best-performing oil is SM. Not to be confused with tying people up and whipping them, which I'm quite sure API no longer endorses.
TOM: So, since your vehicle calls for SG or better, your engine can run on anything from SG through SM.
RAY: By the way, oil designations that start with the letter S are for gasoline engines, and those that start with C are for diesel engines. Those that start with EVOO are for salads and pasta.