Two days ago I was in a store selling Exmark and Hustler ZTR. Since it was my first time in that particular store, I looked around to see what they were selling on the shelves. I was shocked when I saw that some of the various brands of engine oil had an API as old as SH. SH and earlier ones are considered obsolete. The current and most up-to-date oil you can buy as an API of SN. SJ, SL and SM are still ok to use, but you are not doing yourself and your engines any favor if you use anything less than SJ. So you might want to make sure that you are using an oil with an up-to-date API irrespective of the brand and the type of oil used. Btw, in my new mower I'm using (conventional oil) Tutela 10w-30 with an API of SN.