Some oils have different additives which can cause clutch slippage. Yes it is true.
Oh yes, Redracer is absolutely correct!
If ever in doubt? NEVER add any oil into the crankcase that has
"Energy Conserving" written on the bottle. Why you ask? Because these oils contain friction modifiers that will cause clutch slipping over time. These friction modifiers will impregnate the clutch disks and cause them to SLIP when they shouldn't.
Basically all 0w-20, 5w-30 and 10w-30 oils are energy conserving and should not be used in your ATV or motorcycle engine. If you look on the back of the oil container you may notice a Circular information ring that has the SAE weight of the oil located in the center. The information within the top of this circular ring will give you the API service (as an example will say this) SL/CF. And most importantly within the same ring at the bottom will tell you it is Energy Conserving. THESE ARE THE OILS WE DON'T WANT IN OUR ENGINES.
I've spent many hours trying to learn and understand what I need to know about different types of oils on the market today. I found myself not paying enough attention to what oil I was using in my motorcycles. One day I realised the big issue for me is the engine oil is the cheapest protection you can add to your engine that protects the mechanical operation not only in the engine, but also the transmission. So I needed to find out what the different additives were that either enhanced or negated proper lubrication qualities within the engine and transmission.
All I can say is I did myself a big favor by reading and understanding about the different qualities and additives in oil today.
Something you may want to try? Get all of that oil out of your engine, and get rid of that filter. Immediately change up to a Non Energy Conserving Oil. Then run your ATV around for a while to see if the slipping slows down or stops completely? After all....What do you have to loose at this point in the game? The oil and filter will either work, or it won't. If not? Then you'll have to change out the disks?