I use Maxima SynBlend 4 semi-synthetic oil. Works great. I race the WR pretty hard and change the oil & filter after every race. Then i also change it after every 3-4hrs of just general riding if i don't race it. Oil comes out looking new every time.
Personally i'd go with the Rotella T6, but i get a good amount of gift certificates to RockyMountainATVMC for racing so i haven't spent a cent on oil or filters all summer : )