I was given a used riding mower as a gift, and the other day I started to do some of the yearly maintenance as recommended by the owner's manual.

When I got to the part about changing the oil, I saw the words "SAE 10W-40," and I remembered that I happened to have a couple of extra quarts of Penzoil on hand that I had bought from the auto parts store. So I happily used them. Simple, right?

However, upon a closer reading, I see that the manual actually recommends one of the following:

• PLUS-4 SAE 10W-40
• SAE 10W-40 API Service Classification SG or higher

Huh? It didn't even occur to me that there might be different kinds of 10W-40. Did I just put the wrong kind of oil in my mower?

(I should mention that I am a newbie at anything mechanical or motorized.)