I am running Mobil 1 20-50W in the motor, bel Ray primary oil in the primary, and Bel Ray gear oil in the trany

 

Most folks just try different oils in the primary and go with whatever gives the best clutch feel. I've tried Amsoil and Syn3 20W-50 oils, as well as Redline MTL and Formula+. I use F+ simply because it gives the best clutch engagement. I know a lot of people who run Rotella HDEO and Valvoline 10W-40 & 20W-50 conventional oils and they're all happy. Don't know anyone using ATF, but I hear it works well too.

In the tranny, any gear oil will work fine. Considering even the cheapest Super-Tech gear oil is designed to be run for 50k-100k miles in heavy truck differentials, our little HD tranny isn't going to wear out such an oil in the brief year or two it's in there before being changed.
The tranny is only spec'd for a GL-1 lubricant, which many decent synthetic 20W-50's cover. This is why HD says Syn3 in all 3 holes is fine. Putting a GL-3 (Formula+) or GL-5 (gear) oil in there is overkill, but it does the job. Will it add any miles to the life of the transmission? You'd be hard pressed to prove it, but it does give some folks a warm & fuzzy, and since it's their bike that's all that matters...

 

Depending on how noisy your gear box is, you can use 75- up to 140 W gear oil. The thicker the gear oil, the less noise the transmission will make. I don't care for anything thin, such as 20-50 in the transmission, it just causes it to be very noisy. Good luck on your choice. TJ

 

I saw a post to the question of using Mobile 1 75-90 in Harley Davidson transmissions and the mobile 1 guy answered that the GL-5 lube can be hard on the yellow metals such as brass and such and advised not to use it. Anyone else hear of this?

 

Gl-5 is all I have ever used in my dirt bikes and the gears work fine. I had a Husqvarna 250 that was 22 years old when I sold it and it shifted great. Can't see how a top end gear lube could hurt my Fatbob.

 

Refering only to transmission here. They are referring here to a corrosive issue here were the additives in the GL5 attack bronze bushings. Do not think there are any in the transmission. However GL5 is probably over kill. By the way Harley's Formula+ is a heaver weight oil at room temperature then Mobil-1 75-90 synthetic. I had the 75-90 in during the winter and had a few missed shifts going for 2nd. Thought it was the oil. Thru in formula+ . Did not help, it was me all bundled up. I had a quart left of the 75-90 this summer and I thru it in after making the old drip comparison between it and the formula+. My opinion is there is little difference that I can detect in the noise going from neutral to low. I think the FI bikes are a little louder due to the set higher idle (1100) since a lot of owners of carb bike drop the idle lower. As loud as mine is the next time I am going to try the 75-140....... P.S... The noise does not hurt anything.

 

So far I have used only 3 different oils in the primary of my 08 FXSTC. Syn 3, Amsoil 20/50, and Hd Formula+, and in my opinion, the Formula+ is the best of the three. It also works the best in the tranny of my 06 XL. I'll always use Amsoil in my motors and tranny on my FX though. Love it there.

 

Go for the Mobil 1 75W140 for the tranny. I also put a Synthetic Blend Valvoline 20W50 in the Primary. Shifts like butter. Very happy with that combo.