I too use the Castrol Power 4T or RS when I can find it...and its been the best.
Im not saying your wrong or questioning your judgment, I was just wondering what your reasoning is on why its the best. I too am running the Castrol Power1 10w40 full synthetic in my 800EX this season. Its been one one two day trip so far in pretty high heat and dust in a low and slow environment. Temps seem the same, oil pressure light goes out on startup the same, engine noise is possible a little loader but it could be in my head. The only definitive improvement Ive seen first hand is that my fuel economy is better than it has been and also better than others in my group. I rode all day in the heat on technical trails in low with higher than usual RPM and used aright at a half tank. Everyone else was lower on fuel.
To the original posters question, you cant go wrong using a fluid that meets the manufacturers specifications. Im using the Castrol only because I wanted to try it and I was able to order it from Walmart.com with free shipping. Probably wont use it again unless its a deal or something stands out this year showing me its worth it. I have a few machines that I spread the riding out on so I figured that I put about 25 hours a year on each one give or take. So Im ok with just servicing them each once a year. Ive had no problem with Valvoline ATV 10w40 or Maxima Premium 10w40. Both are cheaper and seem to work just fine for my use. I use 80w90 conventional gear lube for the differentials. Ive used Valvoline and Lucas. Cant tell a difference. Also drain those once a year so its overkill on maintenance.
As for your trans if you have a separate one, Cfmoto specs a 75w90 which is only available in synthetic which tells me they want a synthetic in there. They still call for 80w90 in the diffs of those machines so the trans must need synthetic.
NMKawierider gets the moneys with out of his oil by using it multiple seasons. Im more a fan of meets the spec and change it often. To each his own. Just try one out and if you dont like the results pick a different one for the next change. Im considering doing a fill to spill on my next change of the diffs. So far in my 2 services o. Them all Ive gone with the manual fill capacity.