You're wrong.Unome said:
The clutch is swimming in there along with the chain connecting the engine drive and the clutch hub. There's no gears meshing like in a differential.
I put Mobil 1 15w-50 in the engine but bought the magical H-D fluids for the primary and trans since the amount was so small and I didn't have time to research any alternates. I refuse to pay double for "V-twin oil" that is probably the same thing as the stuff I bought at K-mart for $3.99/qt. Sure I can afford it - I just hate being ripped off for $15 because its "specially blended for v-twins." Bull$hit.
The only worry I would have with dumping any 75w-90 in the primary case would be friction modifiers used in a regular gear lube causing unpredicatable clutch performance. Looking at the BelRay site, they have one product clearly labeled for gear assemblies only (gear saver hypoid gear oil) and another for primaries and transmissions (motorcycle transmission oil). The reports of 10w-40 motor oil going in the primary case agree with my eyeball guess of the HD primary oil - it is definitely not thick and stinky like gear lube. Next big service I do I am inclined to buy an extra quart of the 15w-50 motor oil and use that in the primary case while finding some syn 85w-140 lube for the trans.
Unless someone with a better experience posts here first
