I am a big fan of Amsoil. But, any oil will work fine so long as its suitable for motorcycle/atv clutches, and you keep it and the filter changed regularly.
I do have a couple Amsoil testimonys:
I used to have a '99 400 EX, after about 5 years of hard, hard riding, I thought I had bent a shift fork. An ATV mechanic friend and I tore into the engine to replace one of the forks. He was amazed at how clean my engine was, and how little wear was in the cylinder. I had been using Amsoil since after break-in..........the "bent fork" turned out to be noting more than my brand new stiff boots, the first time I wore them I took a big hit from a log, hard enough to bend my shift lever backwards. I bent the lever back into shape and started having trouble getting from 2nd to 3rd. After replacing the fork, I still had trouble shifting, but not while wearing my regular shoes. Solution= keep the boots oiled.
I also use Amsoil in my 2000 silverado, a couple years ago I tried to save $$ by switching brands. I noticed after the truck sat for a couple days, upon startup the valve train would rattle for about a minute. Going back to the good stuff, the noise hasnt been heard again. My best guess is that sitting so long, oil drains from the top of the motor and takes a minute to coat all the rockers after startup. I cant prove its the Amsoil, but I am a believer.
sorry for the book