I use Mobil 1 Racing 4T 10W-40 (http://www.mobiloil.com/USA-English/MotorOil/Oils/Mobil1_Racing_4T_10W-40.aspx) it is pricy but it is great stuff. It also meet all the specs for my Honda PWC and I use it in there also. I have a ton of hours on my KFX 450 and it seems to run fine. I would say I have somewhere between 100-200 hours on my motor of very hard riding. It is great oil but you will pay for it. You can buy it at Advanced Auto Parts.
i use amsoil 10/40 wet clutch motorcyle oil and actually noticed a change in shifting but like the mobil one its pricy but very good.if u look over your fill hole it tells you how much the manufacter tells you put in but it really takes 1qt3/4. the dreain plug is under the bike were the hole is cut out in the plastic shield, the engine and trans are one piece. the fill hole is on the right side on the clutch cover, and the oil filter is next to that