Decided to try Royal Purple syn gear lube in my trans..the store I was at had 75W-90 and 75W-140...I know a lot of people run the 90, so that's what I picked up. Any pros/cons to the 140 over the 90?

 

140 is a little too heavy it might make shifting slow.If you had very high mileage it might help it last longer.

 

I've put the 75/140 in mine in the summer, here in Ga. it gets pretty hot, it worked fine, and quietened the tranny down too.

 

I don't see any wisdom in second guessing the people who built the motor. I'd follow H-D's recommendations for fluid weights.

 

Mine seemed to shift smoother with 75/140 (Mobile 1) than it does with 75/90. Didn't notice any other difference. I may go back to 75/140, its about $6 more per quart.

 

I'm using the Mobile 1 75/90. Works great! Shifts smoother, quieter, and no klunk.

 

Taken from Wikipedia concerning multi-grades:

A specific oil will have high viscosity when cold and a low viscosity at the engine's operating temperature.

The idea is to cause the multi-grade oil to have the viscosity of the base number when cold and the viscosity of second number when hot. This enables one type of oil to be generally used all year, and when multi-grades were initially developed, they were frequently described as all-season oil.

The viscosity of a multi-grade oil still varies logarithmically with temperature, but the slope representing the change is lessened. This slope representing the change with temperature depends on the nature and amount of the additives to the base oil.

 

No harm in using the 140, just more than what they call for. Only downside, as mentioned, is more drag and less fuel economy.

 

I use Amsoil 75-140 Severe Gear Oil in my RKC.

 

I use 75w-140 in my '07 Eglide. Quiets the 5th gear whine. If there is a downside I haven't discovered what it is.