This is the way I understand it: 10W-30 oil in your mower, is exactly the same as SAE 30 oil in your mower, when at operating temperature. The difference is, the 10W-30 oil will behave exactly like a SAE 10 weight oil when cold, which means it is thinner than SAE-30 oil when cold. But this is not a good thing, if the manual calls for SAE 30. Don't confuse the W in 10W-30 as "weight." I believe the W stands for "winter." Nevertheless, when the engine reaches operating temp, even in winter, the 10W-30 is just like SAE 30. On occasion, I've run out of SAE 30 and I've used 10W-30 in its place. I've found that 10W-30 in my Kaw will be consumed much faster than SAE 30, which is what he manual calls for in my Kaw engines. I guess it's too thin during warm up, and therefore burns off. Therefore, I really try to stick with SAE 30. Yes, it seems harder and harder to find nowadays.