I put many hundreds of hours over 10 years on a Yamaha 9.9 hp 4-stroke in saltwater under very heavy use using nothing but whatever oil I could get cheapest, but then I eventually did switch to Amsoil 4-stroke stuff. I will say the engine ran noticeably smoother with the Amsoil, but there was no detectable change in performance. Hard to say if the Amsoil would have improved durability as my engine was suffering from saltwater corrosion when I sold it. Engine still ran and worked perfectly though. Today I think I would go for one of the HDEO oils like Rotella. Again, lower unit I used cheap stuff, but I did have to have it rebuilt after the seal was destroyed by some fishing line. The 15 HP, which is nearly the same as the 9.9 HP, is extremely durable whatever oil you use.
Just make sure you use a good fuel filter. I installed a Mercury spin-on type (looks just like an oil filter) and ran all gas through it. Made a world of difference in preventing carburetor clogs. The passages in those carbs are so tiny that the most miniscule piece of crud will plug up the low-speed jet. Also, add StaBil to all your gas.
