I have a 2008 Ferris 1500Z with a Kawasaki 21HP engine. I usually operate in temps ranging from the 50s to 90s so according to the manual, a 10w-30 would probably work well. I currently am running Valvoline Premium Blue 15w-40 but I also have some Schaeffer's 9000 5w-40 full synthetic that I run in my TDIs that I thought about using. Should I just use a good dino gasoline rated oil or would synthetic be better for this case? I don't put too many hours per year on my mower so I'll probably only change the oil once a year.