I run gear oil in the transmission and formula+ in the primary. The gear oil I use is 75w130 and is synthetic. The trans shifts much quieter and smoother than with motor oil. It is also easier to find neutral.
Some guys use hydraulic oil (auto trans fluid) in the primary but think about it, that's where your clutch plates are...how slick do you want your clutch to be ?

 

In theory ATF in the primary would actually be good for the clutch plates. Automotive automatic transmissions have a series of clutch packs similar looking and function to a motorcycle. They are designed to run in ATF. The problem I have with ATF in the primary is that there is also a big steel chain that I want to keep lubed. Not sure what effect ATF has on chains. I'll stick with the formula + which is only about $9 per Qt at the local HD dealer.

 

Going just from what I've read, compensators don't hold up as well with ATF in the primary.

 

I've also read the same. I experimented with different ATF types and found the primary case was louder, much hotter, and I had basically no friction zone. I'll stick with a good JASO rated synthetic like Mobil 1 Racing 4T in the primary. BTW, the Mobil 1 V-Twin 20W-50 isn't JASO rated, I had clutch slippage when I tried it.

For the transmission use a proper gear lube. I've had great results with Redline V-Twin Transmission w/shockproof.

Now, a tranny lube, that's going to be your decision...

 

Exact set up I use,,,works well for me on my 03 also.....

 

ATF is fine with chains. Jeep has been running ATF in their chain driven transfer cases since the beginning.
The issue I have with ATF is I've seen a few compensators that look bad with ATF.
I'm running whatever 20/50 motorcycle motor oil I have on hand in all 3. It works fine for shared motor/trans/primary setups and I can't see why it won't be fine for separate holes.

 

Redline Shock proof heavy its nice a strawberry milkshake red so if you spring a leak you know exactly what it is ...NEVER RUN MOTOR OIL IN A GEAR BOX ..Ask any HD mechanic how many trannys Harley warrantied when that SYN3 garbage came out .

 

Amsoil tranny.....

 

You can use SYN3 in the tranny but most wrenches will tell you that you shouldn't do it....I'm one of them...go figure.


I use and recommend 80w140 Hypoid Generic gear oil. The 5 speed is an ancient (and very good) design and needs something thick and heavy in there...6 speed is less pre-historic but also likes the thick stuff.

 

GEAR OIL where the gears are
ENGINE OIL in the engine and
PRIMARY OIL in the primary...
You will not retire earlier or get more horse power with any other combination, but you will have lube available anywhere near the nearly 900 HD dealers in North America as you travel...and the bike will run fine