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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.)

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Most oil viscosities come in different additive packages depending on the intended use (ie, low detergent oil, oil for diesel or gas engines etc, specific oil for motorcycles) even though they have the same viscosity rating. That's (partially) what the API rating tells you - for example "S" ratings are usually for gas/petrol engines and "C" ratings for diesel engines. A lot of oils have dual ratings if they're usable for both petrol and diesel engines, but oils designed predominantly for diesel engines tend to contain more detergents than the ones for gas engines.

For a lawn mower that takes 10W40 I'd say that the normal (ie, non-synthetic) Penzoil should be fine (ie, almost overkill). Basically it's more work trying to find a API SG rated oil than finding one with a higher API rating :).

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The SAE grade (in this case 10W-40) only specifies the viscosity of the oil - it doesn't say anything specifically about the composition of the oil or any additives it may have. So for a particular viscosity grade, it might petroleum-based or synthetic (or a mixture of the two), and might have any number of additives that affect various properties of the oil.

I wouldn't worry about it in a mower engine - I'm sure it will be fine :)

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