Whatver you do, don't use the 75/90 or 75/140 gear oils in your transmission. Mobil 1 has acknowledged that they are not good for Harley transmissions because they will deteriorate the brass within the transmissions. Of course, I've noticed most dealerships have started carrying Amsoil and they carry the Amsoil 75/90 gear oil which is the same as the Mobil 1 gear oil, API GL-5 type of oil.

 

Where did you get that information from? I've been using Amsoil 75w110 gear oil in both my bikes with no problems.

 

What brass?

 

http://www.mobiloil.com/USA-English/...ow_Metals.aspx

Question: Does Mobil 1 Synthetic Gear Lube Corrode Yellow Metals?Is Mobil 1 Synthetic Gear Lube LS 75W-90 gear oil OK to run in Harley Transmissions? The big buzz on the internet chat rooms is that it corrodes "yellow" metals, such as brass and copper. Is this true? Please answer THIS question.-- Michael Carravetta, Janesville, WI

Answer: Unless the transmission specifically recommends an API GL-5 fluid for your transmission, it should not be used. GL-5 gear oils are designed for gears that require a high level of EP (Extreme Pressure) additives to protect gear teeth under extreme stress. Unfortunately, these same additives can be very aggressive toward “yellow metals “and are generally not recommended where copper, brass and bronze components are used.

 

Exactly ! . . . . or you can get the Royal Purple 20 - 5o motorcycle oil. Or ANY FULL synthetic motorcycle oil


.

 

i did a bunch of reading on this. it is not just mobil1 that will deteriorate the 'yellow metals', it is all gl5 rated gear oils. that includes amsoil, royal purple, etc. however, there are additives added to this oil to mitigate the corrosive properties, and because of the frequent (20k or less) change interval as compared to regular automotive transmissions (which can go 100,000 or more) your safe.

i went to walmart and looked at all the gear oil they stocked. it was all gl5. same with autozone. so if you don't want to use gl5, good luck finding any gear oil that isn't gl5 rated. that includes the golden child amsoil as well.

 

There might be some truth to the corrosive effects of the synthetic gear oils. I used Mobil 1 gear oil in my 04 Sportster. All the brass rivets in the clutch plate let go around 18k miles. I put in a new clutch and switched back to Formula +. Then in my wife's 06 Sportster, the stator went out. I had been using the Mobil 1 in there also. NO MORE. Can't prove it was the synth gear oil, but I am suspicious.
I traded my Sporty of my current Fatboy. I use Spectro Platinum 6 speed gear oil in the tranny and Formula + in the primary. No problems with 22k on the ticker... I do use the Mobil 1 20W50 motorcycle oil in the motor.

 

If you actually check Mobil and Amsoil's recommendations for Harley's, they say use 20w50 in all applications. Neither recommends using their gear oil, that's what I'm saying.

Personally, I've switched back to just using Formula+ for primary and tranny and I still use Mobil1 20w50 for my engine. The only reason I switched to the Formula+ is because of finding neutral. It's just easier with a thicker oil than 20w50.

 

I use Mobil 1 20w50 v-twin in the crankcase.

Royal Purple 75w90 gear oil in the transmission.

Redline MTL in the primary.

 

part of your problem is using gear oil in a sportster. the transmission and the primary share the same sump. you wouldn't use gear oil in the primary of your fatboy, why would you use it in the primary of your sporty? i always used sportrans in the sporty which has now been replaced with formula +.