I use amsoil for 3 reasons: (1) they give you cool stickers, (2) everyone else I ride with uses it and (3) it's widely available if you have to buy it on the trail (we backpack most trips).
I have about 7,000 miles on my Polaris. The engine was rebuilt for reasons not related to oil, so I have no idea if it is great oil or not. Guy in my group put 12,000+ so far on his Cat 700 before selling it to a relative so is still using it, but supposedly that thing will run on Cat pee so who knows.
There doesn't seem to be any conclusion evidence to support one oil over the others, and if there is, someone else will argue with it.I will probably switch to Mystik semi in the next sled. I have yet to hear of anyone having trouble as a result of it, and it's cheap. Seems like a no brainer to me. I would just have to plan ahead to make sure I could carry enough on a trip so I don't have to mix in other oils.
In the end, the cost difference isn't all that much to the total price of riding, particularly with the newer engines getting more efficient.
On another site, I've seen some quotes from a reputable Polaris engine builder named Indy Dan who says to never use synthetic oil because it doesn't stick to the internals of the engine (bearings, seals, etc.). That has made me wonder if semi or even non-synthetic is a better way to go.
