Know it really does not matter but you are light on that inner primary bearing. And also too heavy for flushing those small needle bearings on the transmission input shaft where it runs inside of the output shaft.
Strike three.

 

 

Hey RipSaw, any suggestions, I like your input about being light and heavy. So what is the in-between? I have been running 10w40 in the primary but willing to listen to what you have to say.

 

I am not trying to be an expert here and I am not a tin man.. Since the FP posted what he was using, I was just posting what Harley engineers say. For a while, they even called out SYN motor oil for primary and transmission (20W-50). Then they went back to their 50W non-detergent non-synthetic gear oil as first choice. I assume because of the number of warranty issues. Still, list SYN as a 2nd choice.

Motors are happy with 20W-50. Or non-systhetic 50W. (I use Mobile 1 15W-50 since it falls in the correct range in my opinion ..Ha..but I poke around and never turn over 3200 or so) The only source I have for the 50W non-detergent is Formula + for transmission and primary

In between, 75W-90 Synthetic gear oil is close to 50 they say but it does have EP additives and you do not want that in primary on an older bike with brass in them and sure not healthy with a wet clutch.

As I understand it, 75w140 is just as thick as 75W90 but at higher temperatures, it holds up better then the 90 and it also has more additives for heavy duty use?
I have heard also that the 75W gear oils are equal to 20W-50 motor oils because they are measured differently. Over my head here and why I just stay with what the Harley engineers say.
Easier that way and I keep things forever.