It's important to use oil rated API SE, SF or SG as your manual says, because the newer API ratings like SJ & SM have additives to increase fuel economy in cars which may cause clutch slippage in motorcycles and ATVs. Oils rated either API-SG or JASO-MA (or both) are appropriate for use in an ATV. API-SG was the last automotive rating for motor oil considered appropriate for motorcycles. JASO is a newer oil rating system developed by motorcycle manufacturers due to the fact that API ratings after SG should not be used in motorcycle and ATVs.
There are a number of old topics on the forum where oil has been discussed extensively. You can find some of these in the archives by using the search feature, but here are a couple which include some links to web pages with more information:
& Motorcycle Oil Wisdom