This one is hotly debated. I used to have one and I believe the manual said to use a synthetic blend oil with a weight of 5W-20, though the argument can be made that government regulations, and not what's best for the engine, ruled the day on that recommendation. I also used 5w-30 oil from time to time, which is the preferred option among some farmers. I didn't notice much of a difference in performance between the two honestly.