That really should be amended to say don't use heavy duty hypoid or differential gear lube in your primary. And never use any oils with friction modifiers (which many hypoid gear oils contain). What I was referring to is motorcycle gear lube for wet clutches...I run 80w Golden Spectro in mine...have run motor oil (10-40), Type X, the old HD chaincase lubricant....no real difference in any of them as far as I am concerned.
10-40w motor oil is about the same viscosity as an 80w gear oil...
DISCLAIMER; Always refer to your service manual for specific model recommendations and service procedures.
GOOD INFO!!!!!! SAMF