I have a 90 hp Yamaha 4 sroke. I was about to start buying oil from somewhere like CTC, I figured a quality 10w30 like Mobil One would be fine. Doug Nagtegal from Bay City Marine spoke at our club in the spring, one of his recomendations was to stick to the Marine grade oils that are sold by Merc, Yamaha,Honda...They are formulated for the specific conditions of marine use...I decided to stick to Yamaha 10w30 4 stroke oil. Though I'm sure regular good quality 10w30 would be fine, why mess with a $10,000 Outboard...