Jerry sez:

what is the advantage of ignoring H-D and going to heavier
(75W90) oil in the primary case for clutch and primary chain
lubrication?

ooooooooooooooooooooooooooooooooo

Gear oil and engine oil use different "weight" scales.

http://en.wikipedia.org/wiki/Gear_oil

API viscosity ratings for gear oils are not directly comparable with
those for motor oil, and they are thinner than the figures suggest. For
example, many modern gearboxes use a 75W90 gear oil, which is actually
of equivalent viscosity to a 10W40 motor oil.

There is a much better reference out there, if you want to take a look
for it. Gear oil and motor oil have different chemical additives,
viscosity is important but not everything.

Skinny Kenny