I've used cool power 30% mostly for about a year now. Get the synthetic and it won't brown up your muffler. If your just doing hovering and forward flight the 15% is fine, you'll get a bit more power from the 30%, you'll just have to richen your needle a bit.
As for why, that's the best my hobby store carries, sometimes they are out and I'll have to use wildcat, which runs fine also.