Thanks to all for all of the info on HDEOs for motorcycle engines. I have started using Rotella 5w-40 in all 4 of my ATVs and am happy with the price and performance. One question I would like to have some input on: on the 2 air cooled ATVs is Rotella 5w-40 "synthetic" OK for summer usage? I live in Texas where the temps regularly get above 100 and want to make sure these 2 are getting the best protection. We ride long and hard... Call me strange, even though I know that the 5-40 is not a true PAO synthetic I want to use a synthetic labeled oil. Was looking at the RP 15-40 synthetic HDEO. Any tips? Thanks!