Every once in a while there is a raging (heated) discussion on TheHullTruth.com (and sometimes TidalFish.com) about using synthetic oil in the Yami 4-strokes.
Bottom line of all these discussions is that Yamaha doesn't recommend it, and those who are using it are doing so at their own risk.
I agree that as long as the oil meets the API & SAE requirements, you should be able to use it and not have any warranty problems. With $18k of engines hanging off the back of my rig, I've never gotten the nerve to use anything other than Yami 4S oil.
A consideration for me is that both of my F-115s are getting a bit of sodium in the oil. I started oil analysis a couple of years ago when I had one engine show signs of the fabled F-115 "Making Oil" problem. I expected fuel dillution, not sodium (probably salt water intrusion). I now change the oil every 50 hours (or twice a season, whichever is shorter), and do oil analysis on the oil coming out each time. Although these two F-115s have now crossed the 500 hr mark and are running beautifully, in light of the oil analysis, I'm going to keep running Yame 4S oil, and giving Harvey (@ Tri-State Marine in Deale, MD) the oil analysis sheets that come from Herguth.