I just changed my oil today and went with Amsoil @ $8.95 a quart. I was using Maxima full synthetic in the beginning and recently using Castrol R4. With the R4 and Max. the bike was shifting clunky and not as smooth as I would of thought with Synthetic. I had the R4 in for about 2000 miles and then went with the Amsoil. Totally different bike no kidding. I wish I would of been using it the whole time. I am now an Amsoil man!