I have gone through this with a 2005 F250 on a 2320. It is unburned fuel leaking back into the crank case. If you google "making oil " you will learn that it is not only Yamaha, It is an issue with 4 strokes. I have spoken w/ Yamaha reps and their advice was running at higher rpm's. I have bumped my cruising speed from 4800 to 5000. I had been cheating some on fuel w/ regular gas. I now run no less than 90 octane . I also run WOT for 5 minutes or so on each trip when the seas allow, which was another suggestion from Yamaha rep. This solved my engine from making oil. I did the above mentioned checks of thermostats, injectors, etc. I live and fish on Delaware Bay in Jersey. I have seen commercial crabbers who run all day at little over an idle have oil run out of the dip stick. I think it is all BS, the industry should be able to build a 4 Stroke w/o this issue. The 4 stroke is the same thing that's under the hood of your vehicle and that motor does not make oil. All Yamaha wants to tell you is, that you did not break it in properly so they can void warranty on a faulty engine.