5w40 is a European oil mostly used in high end German cars and most new diesels and yes, it's synthetic.
I'd go with 10w40 and at a minimum make sure it's a oil designed for ATV's.
No offense to the Amsoil, synthetic and all that other expensive stuff, but conventional oil works fine and has for 100 years. The most important thing, regardless of what you use, is to keep oil clean and filter replaced with a quality product, and you'll be fine.
If all those others were that much better, don't you think everything motroized that uses oil would run it?
A forum community dedicated to Off Road Powersport owners and enthusiasts. Come join the discussion about performance, reviews, classifieds, troubleshooting, maintenance, and more! Open to anything that slings mud Including Yamaha, Honda, Kawasaki, and Suzuki.