I have a 2002 Yamaha F115 Four-stroke outboard and to date I have used conventional 4-stroke oil. In all my vehicles, I use synthetic oil exclusively because I believe it provides superior flow at low temps and better protection at high temps. Oil cost is not a concern. I would like to switch to full synthetic in my Yamaha, but from what I understand Yamaha doesn't recommnend synthetic oil solely because it hasn't been tested in their engines. I can't think of any reason why synthetic wouldn't be a better choice, but I have a nagging doubt based on the fact Yamaha doesn't recommend it. Anybody here running synthetic in their Yamaha or Merc 4-strokes? Experience? I appreciate any comments and feedback. Thanks.