well, i've been using shell rotella t 15w-40 diesel engine oil. it is jaso rated for wet clutch use and runs about $14.00 for a gallon jug of it. never any issue with clutch slippage or the like. been using it for nigh on 7 years now i guess. i have always, and continue to do so, change the primary oil every 5k miles. same time as the engine oil. did the clutch adjustment at the same time. with the hydraulic clutch, don't have to adjust, but still change at the 5k interval. i do let my tranny oil go for the full 20k. figure if it can last over 100k in a car, then it should be able to withstand the 20k interval the hd engineers spec.