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Use only SAE 30 oil in small engines!
That is what is shipped with every mower.
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What’s wrong with 10w30?
Kohler wants you to use 10-30. Every one else 30.
NO! Do NOT use only SAE 30W motor oil in small engines.
Consult the owner's manual for YOUR small engine and use the weight of oil specified therein. If the engine was manufactured in the XXI Century, there's a very high probability that the specified oil will be of the modern multi-weight variety. All of mine (two B&S, a Honda, and a Tecumseh) use 5W-30, as specified by the manufacturer. But this has nothing to do with YOUR small engines; consult YOUR owner's manual(s) and operate accordingly.
That is what is shipped with every mower.
My son found a way to blow up my 27 year old lawnmower that was in basically perfect condition so I purchased a new mower with a Honda engine. I was shocked to see that I had to use 10W-30 automotive oil.