Primary/trans oil

engine oil v gear oil

Actually, 20W-50 is as thick or thicker than 75W-90 gear oil. SAE Engine oils and SAE Gear oils are on a different scale. Mainly, because running gear oil in an engine is not good.

HD Sport Trans was close in viscosity to a 40W engine oil or an 85W gear oil. Many believe it was just a 10W-40 engine oil with a bit of EP additives.

HD has a replacement for the Sport Trans. I believe they call it Formula Plus. It's a bit thicker than the old Sport Trans, favoring the transmission over the primary.

If you don't want to us the HD, there are several good gear oils out there from Amsoil, Mobil 1, Redline etc.

I'm running a true Synthetic 75W-90 in both of my Sportsters.

Honus402, Are you using synthetic 75-90W in your primary/trans? It sounds like you are. Have you experienced any clutch problems or improvement in shifting etc? Thanks

ctd said:

My Sportster runs a common oil in the primary/trans. The manual says to use the Sport Trans oil (20-50W?) but I notice on the bikes with separate compartments, a heavier gear oil is used in the trans. Some run Redline MTL in the primary. Will the MTL work for both the tranny and primary?

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MTL is a 70W80 GL-4 Gear Oil (SAE 5W30/10W30 engine oil viscosity), probably not what you want to be using in your trans. I use all Redline lubes in my TC, but unlike your Sporty, I have no common systems. If I were you I would be looking at Amsoil MCV-20W/50 oil. It has a JASO service grade among others and would probably be ideal for your Trans & Primary.