I think if I had your atv , I would run the OEM oil , but don't think you have to
even though all I have every used in the last 30 years plus is Shell Rotella in everything from dozers to my trucks , atvs , lawn mowers , log splitter , etc , etc -------as far as the certification on the bottle goes , it doesn't bother me that the conventional Rotella doesn't have the spec suggested by ATV manufacturers , I was told a long time ago by a Shell Rep. , that Shell is required to pay for the testing of that cert. , so they don't certify "" every "" one of their blends to be able to wear that cert. spec on the bottle , but it doesn't mean that their oils with out the spec doesn't meet the spec , for many years while I was service manager of a equipment rental store with multiple locations , under the recommendations of the Rep. all we used was conventional Shell Rotella 10w30 in every gas engine and 15w40 in every diesel engine without any problems , I can't see paying extra for a cert.
a interesting thing about synthetic oil which I believe to be true , a lot of people think synthetic oil is man made , it is really just oil that man manipulated , say there are 3 types of atoms in oil , A,B and C , A being the best quality atoms , they process the oil and remove all the A's ---often wonder where all the lesser quality atoms go to , maybe Circle K Golden State motor oil