Dear Tom and Ray:

Remember in the 1950s, a quart of oil would have API designations like MS, DG,
etc? Now I see letters like SJ, SH and SG. Do these letters mean different
breakdown temperatures or different additives or what? -- Bob

RAY: Yes. The letters refer to a set of voluntary standards administered by the
American Petroleum Institute (API). They cover various oil properties --
lubricity, breakdown temperature, particle suspension, etc.

TOM: Every so often, the API -- in consultation with the oil industry and car
makers -- raises the standards. And when they do that, they assign the new
standards a new set of letters.

RAY: For gasoline engines, the first letter is always "S."

TOM: Wait a minute. Why is it "S" for "gas"? Wouldn't it make more sense to use
"G" for "gas"? Am I missing something here?

RAY: That's a closely held trade secret. Anyway, the second letter goes down the
alphabet as the standards get more current. So "SA" was the first standard. Now,
the current standard, introduced in 1996, is "SJ."

TOM: And for diesel engines, the designations probably all start with "Q," right?
RAY: No, "C."

TOM: Ah! Remind me to discuss the sorry state of the American educational system
with you someday.

RAY: "CH-4" is the current diesel standard, having been introduced in 1998.

TOM: The other question we get about these letters is: "Can I use an oil with an
outdated standard in my car today?"

RAY: The question usually starts out: "I just found a bunch of 'SF' on sale for
13 cents a quart (or I just inherited a case of 'SF' from my late Uncle Marty).
Is it OK to use it, even though today's standard is 'SJ'?"

TOM: And the answer is yes, it's fine as long as your owner's manual calls for a
standard of "SF" and not something better. If it was good enough when the car was
new, it's good enough now.

RAY: But if your owner's manual specifies something more current (in this case
"SH" or "SJ"), then use Uncle Marty's old oil in your lawnmower ... or on your
salad.