Originally posted by skthom2320: I use Mobil 1 15-50 (in the red cap) from Wal-Mart. It fits the bill for what is called for in the manual and it is a synthetic. I have to agree with this! I'm on my 2nd Yamaha Thumper and have used nothing but Mobil 1 15w-50 (in the red cap) in both of them. No problems what so ever.