i think the last time i used 10-40 mx4t was 3 years ago. i had clutch slip, i changed my springs after i had used castrol syntec or something. so the last 2 seasons i used mobil 1 20-50 v-twin oil. hey its a v-twin, its synthetic, and it won't make my clutch slip. but just for poops and giggles i check the service manual before i head out to autozone. it reads to use honda gp-4 or something close to that in a 10-30 weight. i find the racing 4t mobil 1 synthetic, next to the v-twin mobil 1. the v-twin is for air cooled bikes.... so i buy the racing 4t @ 8.89 ea. hey nothing is too good for my bike. and 7! quarts of motorcraft synthetic blend 5-20 for my truck @ 2.69 each. ok now mind you i have a full barnett clutch and spring with extra oil holes milled into the basket. but before i change my oil i come here and check with my prodigies before i put this racing stuff in. come to find its mx4t of a couple years ago, cool. so i just had my clutch slave cylinder bled out and refilled 2 months ago. guy says you're clutch is wiped out. i told him nope thats the way it is since i went to the barnett. just tough to hit neutral thats all. ok i change the oil and go for a ride. remember i have been using 20-50 since the clutch change. well my friends she is shifting a whole lot smoother and the motor is making less noise than with the 20-50. this is only my observation. if you want to use royal purple, amsoil, rotella, or even jed clampetts bubblin' crude use it. but i saw a bunch of posts where spike said stick with 10-40. which i used before a full clutch change. now i'm back to it and my clutch is 2 year old and working better than when i first installed it. good quality oil comes from plenty of manufacturers. if somebody will sell me another top qualiy synthetic brand cheap, i'll try it. but today my best choice was $8.89 a qt. hey once a year change for triple duty this oil does and how much i love this bike 5 years later, suck it up. like i said motorcraft synthetic blend for the truck was 2.69. i like my truck, i love my bike. i'll try to keep you guys posted on the long term of this with my modified clutch and basket.