I am assuming the transmission and the primary share the same lubricating oil.
Is this correct?

 

Yes. You fill it in the primary.

 

1 quart, I run Mobile 1 V-twin 20w 50, https://mobiloil.com/en/motor-oils/m...motorcycle-oil

Mobil 1™ V-Twin motorcycle oil



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• 20W-50

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17 out of 1894%of reviewers recommend this product.
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Full synthetic offering outstanding protection

Mobil 1™ V-Twin 20W-50 synthetic motorcycle oil is formulated to help offer outstanding protection in four-cycle, V-Twin type engines – particularly those that are air-cooled and tend to run hotter than other types of engines.
Mobil 1 V-Twin 20W-50 engine oil is designed to help provide the proper frictional characteristics necessary for wet-clutch common engine/transmission systems.
The unique, synthetic structure of the Mobil 1™ motorcycle oil formulations offers advantages beyond conventional motorcycle oils of similar viscosities.
Please reference our Mobil 1™ Product Guide for a quick look at specifications including zinc and phosphorus, as well as manufacturer approvals for Mobil 1 motor oils.

 

I never see anyone recommend Syn3 from Harley. My buddy works at dealership and swears by it. Any thoughts/opinions on it vs. the mobil 1?

 

Wouldn't you want to use the Mobil 1 gear oil for the primary and tranny?

 

Not from what I have read.

 

I've run Amsoil 20/50 V twin in both holes for nine years with no problems.
I think the HD stuff is made by Citco then HD doubles the price because it's in their bottle, why do they recommend it??? cause they're makin money that's why!

 

Not according to Red Line Oil


https://www.hdforums.com/forum/sport...-answered.html

Here is Red Lines official response -

Thank you for contacting Red Line Oil, in the shared Sportster primary/transmission the 75W90 is recommended, a product that has been used extensively and performs well.

The 75W90 is the product we have have used for a number of years with very good results in Sportster/Buell transmissions, matching the viscosity and type fluid called for.
The 80W Motorcycle Gear Oil is a lower viscosity than called for, the ShockProof gear oils can due to their unique characteristics could cause sticking of the Harley clutches.
The 75W90 would be preferred and recommended over 80W Motorcycle Gear Oil in these applications.

There can be issues with some GL-5 gear oils in certain applications. Some can be corrosive to brass, bronze or copper, not an issue with our GL-5 gear oils, they don’t cause corrosion, pitting or metal removal. In a synchro equipped transmission the friction modifiers contained in many gear oils can make them too slippery, though not the case in a motorcycle transmission with dog rings.


So if you want the right viscosity and dont want to worry about it damaging anything, go with Redline http://www.redlineoil.com/product.aspx?pid=133&pcid=4

 

Nice call

 

This is what I thought as well. I have read many threads stating they use synthetic 75w90 gear oil. I just bought some Mobil 1 75w90 but haven't changed it yet.