Automatic Transmission Fluid (ATF) really does not belong in a Harley transmission. I wouldn't even consider it in the transmission. I believe it would destroy the transmission rather quickly. It would be dead before you know it.

The transmissions really should receive gear oil and GL5 at that. Before SYN3 and Formula Plus.....we had transmission fluid. The HD transmission fluid was nothing but a semi-synthetic GL5 gear oil.

 

Thanks for the info. I also gather you wouldn't use it in the primary either. Use diesel oil ?? The reason I ask is that a friend told me using ATF in the primary is better than anything he has ever used. Just wondering if he knows what he is talking about. Claims it shifts smoother.

 

Syn 3 will work fine. There are plenty of oils that will do the job for you, it depends if you want the same oil in all 3 holes. AMSOIL is great, Mobil 1 is also a good choice, both rated higher then HD syn 3

 

I might would consider using ATF in the primary as ATF has some decent qualities towards the clutch discs. I use to run nothing but ATF in the primary in my old Shovelheads with pretty good results.

ATF probably works fine as it did in the Shovelhead days. However, I don't know if it has the right viscosity when it comes to keeping the chain lubed and the primary bearing lubed as well. I also don't know if ATF has the ability to keep things cool in there either. Rare, but can happen.....motor oil can leak into the primary.....and vice versa. I certainly wouldn't want ATF to leak into the engine and get mixed with engine oil.

15w40 diesel oil is inexpensive and can be found almost anywhere. It is about the right viscosity and it lacks "Energy Conservation." I usually run 15w40 in the primary for about 7-10,000 miles or yearly. I've run many different oils in the primary starting from 5w30 all the way up to 20w50. What I found out is that most oils below and including 10w30 contain "Energy Conservation" and that should be avoided as I encountered clutch slippage. 10w40 seemed to work with no ill effects with no slippage. 20w50 worked alright for most cases....except in really cold weather. Once it heats up.....20w50 works just as good as the others with no ill effects.

I have a diesel tractor......and always have 15w40 available on my shelves. I don't own anything that uses 10w40. Besides, 10w40 is not readily availble in stores as others anymore either as most cars do not use it anymore. The only thing that uses 20w50 are my Harleys.....and I like to reserve that for the engine only.

15w40 is a just about where it should be when it comes to keeping the primary working right. The best thing....it works just as good as others and is usually much less expensive.

 

Thanks for all the info. I will try the 15w40 in the primary.

 

I used to use Syn 3 in all three holes. Decided to upgrade to Mobil one in the motor, Redline Shockproof in the trans, and the HD forumula Plus in the primary.

I think the biggest advantage to using different earl in each hole is that it helps you diagnose a leak. In fact, I have a red drip on my garage floor right now and therefore I know that I have a transmission leak. If I were using the same earl in each hole, I think that it was the primary leaking as I just had the inner/outer off a gew months ago.

 

Now there's a reason to switch I can buy into. Finally.

 

If you are going to keep changing the fuilds as called for in the manual, I would go with the syn 3. If you want to get a few more miles out of it between changes AMSOIL is the choice.