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.

ORIGINAL: bigbean78

...Any pros/cons to the 140 over the 90?

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.

I found in my 2007 that the 75W-140 works best, even here in "cool" NH. I switched to 75W-90 in the fall, but thought it ran best with the 75W-140. Smoother, less clunk!

Thanks for the help, fellas...I'll probably stick with the 75W-90 and see how that works out for me. I thought the 140 might be a little heavy...

Excuse any mis-information here, as I am not an engineer (at least not in a mechanical or hydraulic sense). Isn't it true that a 75W-140 gear oil is still made from 75W base stock, but has additives in it that extend its viscosity index (i.e. keep it from thinning out) up to a point similar to a 140W base lubricant at higher temps. What this tells me is that the base stock/oil does NOT get any thicker even at higher temperatures; instead, it merely maintains its viscosity or keeps from thinning out. Ultimately, this means (at least in my mind) that a 75W-140 oil is no thicker/heavier than a 75W-90 oil, at least not within normal transmission operating temps. No oil can magically get any thicker as the temp goes up;its additives can only keep it from thinning out.
At least that's how I see it.

Or you could use Amsoil 75w-110....

I have been using 75w-90, but am thinking of going to 75W-140. My 2006 Street glides still "clucks" a little and I want to see if this will change things...

I have heard people that switched from Syn 3 to a different gear oil, said it was smoother and quieter shifting.

Just to give you some numbers, here are the kinematic viscosities at 100C of the various fluids:
Syn3-20.3
AMSOIL 75W-90 - 16.7cSt
AMSOIL 75W-110- 20.4cSt
AMSOIL 75W-140 - 25.7cSt
AMSOIL MCV 20W-50 Motorcycle oil - 20.1cSt

Just something to compare. Shows the 75W-90 product runs thinner than the Syn3 or MCV, but is also rated a GL-5 gear lube whereas the oils are GL-1's.

runnin 75/90 but i think i'm going to up it to 140 for the summer down here in Florida

I recently changed the tranny fluid on my '02 bagger to Wal-mart's 75W-140 synthetic tranny fluid. Shifting is now click-click, where it used to be clunk-clunk.

ORIGINAL: bigbean78

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?


I installed the Max-Gear 85w140 last summer in an attempt to quell the clattering of my '07 tranny. It helped, but not as much as IDS, making the heavy oil probably redundant. When I change again I'll probably go back to 75w90, which I used in my RK for >100k miles with no problems. I think the heavy oil is overkill and probably will hurt gas mileage marginally by increasing drag on the tranny.