it will take 10w40 oil designed for WET clutch approved spec's, has to be wet clutch approved oil
and just in weights that work in the temps you will run things in
I personally run AMsoil 0-40 4STROKE oil(and plow snow often in winters and in temps from minus 30 to 100+ in summer time)
and have for 20 yrs now on my 400, and she runs like a new yet
but there are ton sof good oil on the market today to pick from!
I will add this, and NO dis respect meant!
OIL really shouldn't be going more than a yr before being changed even on low use motors, and the reasons is, condensation and temp swings and oil just breaking down
its your atv and you can do as you wish, but very few folks will recommend going YRS without an oil change, even on low use machines, its sort of asking for a possible issue!
like they say, some folks buy lottery tickets and win, some DON"T, so just cause things seem OK< doesn't mean they are!
oil is rather cheap insurance of long life to any motor!