using my 01yz250f as the omnios oil mixmaster of DOOM, i can say that Exxon Superflow (with coupon and rebate at pepboys for 79 cents a quart) is without a doubt the BEST bang for the buck. now, i've only tested the 20w50 SL version, but i'll put some faith in the 10w40 SL working as well. i have Texaco Havoline 10w40 SL in it right now and seems to be fine. my new 06yz250f just got the original oil swapped out after 0.9 hours on the motor with the havoline 10w40 SL. you can get 5qt jug of it for under $7 at wallyworld.
if you are a "local" pro, i would HIGHLY advise you not putting 5 hours on the oil before it is dumped. i would go so far as to say, you need to dump it after every race weekend or long practice session.
being in Idaho, you need to look at cold weather ambient temps to see what you need to run (if you even ride in the winter there). it was 77F here yesterday.