20w 50w dino in all 3 holes for 10 years and 40k... change often.. no syn for this old schooler

 

 

Here’s what I use, not that it matters. A quality fluid made for the application at hand is all you need.

 

Here is the way I understand multi-viscosity lubes, I am confident if this is wrong I will be corrected. Whether you choose 75-90 or 75-140, the 75 part means that neither oil will get thicker when cold than a 75 wt lube at that same temp so both lubes start out the same when cold. When hot the 90 will be no thinner than a 90 wt at a given temperature and the 140 will get no thinner than a 140 wt oil at that same temp. With both oils you have the same flow characteristics when cold but but the 140 is higher viscosity when hot than the 90 wt at the same temp. Both .oils get thinner as the get hot but the 90 is thinner at a given temp than the 140 oil. If you equate higher viscosity with greater protection of the gears, the 140 will provide more protection (film strength) when hot. That is why I choose 75-140.

Thicker oil has a price, however. It uses more power. Think in terms of stirring a glass of water or with the same spoon, stir a glass of pancake syrup or a milk shake. The thicker fluid requires greater effort (power) to stir. So decide what you want, a better chance to win a stop light drag race or quieter, smoother operation from your transmission. It appears that Harley would go with the 75-140 since they came out with their own 80-140. $20+ for the Harley oil is bit rich for me. Any of the 75-140 gear oils from the major names should be fine and less expensive also.

I have read that some gear oil additives will attack "yellow" metal, I assume this is probably brass but may also include copper... as in our stators. So, don't try gear oil in the primary. Stick with engine oil (suitable for wet clutches) or primary specific lube for the primary.

 

My experience as well

 

Spectro Heavy Duty Platinum 6-speed Transmission Oil 75 - 140 , use their primary lube in my primary as well .