I've always used Honda 10w40 synthetic blend in my GL1500 and now in my GL1800. There are many oils out there that can be used as long as they meet the requirements listed in the manual. Some guys run full synthetics like Mobil 1, Amsoil, Rotella and others. You just need to be carefull of oils that have molly in them because some report that the clutch slips with oils that have that additive. So choose your oil, change it and the filter and go ride. It's a beautiful day.. Be safe..