I was leaning towards the Mobil 1 75w90 for the trans, but I found these two comments from Mobil 1 engineers on the Mobil 1 website in the "Ask Mobil" section regarding use of this oil in the trans. It seems like they don't recommend it. Also, when you go to the Amsoil site, they don't recommend any of their gear lubes in the trans either, but rather their 20w50 synthetic oil in all 3 holes.
So I am confused, it seems like the engineers at both Mobil 1 and Amsoil recommend using their 20w50 synthetic motor oils in all 3 locations.
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.
Question: Sorting Out the Lubes for a Harley-Davidson Dyna Wide Glide
I have read your question and answers regarding oils in a Harley with separate case, trans, primary. I have been using Mobil 1 V-Twin 20W-50 in all three holes. I am all about optimal product use and performance. Aside from 20-50 V Twin in the engine from what I have read it seems like Mobil 1 Racing 4T 10W-40 for motorcycles would be better in the primary, and 75-90 gear lube would be better in the trans. Is this correct? 2002 Harley-Davidson Dyna Wide Glide. Also I see you have a 75-140 gear lube. The manual does not specify weights of oil to use and it's one of Harley's marketing schemes to make you buy their oil.-- Michael Lawton, Howell, NJ
Answer: We recommend that you keep doing what you're doing and use Mobil 1 V-Twin 20W-50 in all three applications.
By using a premium synthetic like Mobil 1 you are already using a high performance lubricant in your bike. Using Mobil 1 Racing 4T 10W-40
in your primary is an option but we would not recommend the use of an API GL-5 gear oil in your transmission. (Also see the question
on API GL-5 gear oils in transmissions.)
Recommended use of 20w50 synthetic motor oil for engine, trans, and primary: