Here's a weird report. I have a 93' 150hp Evinrude with a jet. The motor is a fuel/oil hog so I always used cheap stuff and it ran great. A couple years ago my conscience got the best of me and I started using Penzoil semi-synthetic oil. Not long after that, I noticed I was losing "r's" at high end. I never associated it with the oil though. I though oil is oil, right! Well after a couple of years of being unhappy, and several trips to the local motor dude (very good mechanic) we tried different plugs, idle settings, flywheel/timing changes and it really never changed the problem. The average guy wouldn't of noticed it but I know my boat. Well, after the last trip to the doctor in February, we were all stumped. It wasn't the electronic "brain" either! So the owner of the shop who is an oldtime "oil-head " (2-stroke racer for 35 years) asked what oil I was running and I told him. He said some motors were designed to run natural oil only, not synthetic (very slippery?) and maybe I should try it. I thought, "no damnit, oil is oil!" I went crabbing the next weekend and I ran low on oil so I had a quart of standard Castrol outbaord oil in the boat for emergencies. So I poured it in the resevoir while at the take out. The next day she ran GREAT!!!!! I was sure it wasn't the oil, probably the 4 foot chop that I pounded in on, and it wiggled some carb adjustement, but it has continued to run excellent (while "knocking on wood", I don't want to jinx it). SO here's one for the weird file. I don't know, but I'll keep using straight 2-stroke now!