Is 20w50 Synthetic Oil correct choice for the Primary/Transmission in a Model 48? I have used this in my 2003 RK and it has worked fine. I see mention of
99896-88 Sport-Trans Fluid is this needed or necessary? Is it any better?
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Oh boy, an oil thread.

The straight-up honest answer is that just about anything up to and including 50 weight motor oil will be fine in a primary. Harley's own "Formula+" primary lube essentially tests out as a basic 20w50 motor oil.

Harley's own Syn 3 works just fine in a primary too.

So does Redline's MTL.

So do any number of 75w90 gear oils (note: 75w90 gear oil is essentially the same viscosity as 20w50 motor oil, the two specs use different viscosity numbers).

Basically, you can run even a simple GL-1 rated oil in the primary and should see no problems from it. A GL-3 or GL-4 rated oil should do slightly better, but overall the primary really isn't that picky.

The only caveat is that you should NEVER run a GL-5 rated gear oil in the primary unless it explicitly states that it's OK for limited slip differentials. A GL-5 oil not rated for limited slip differentials can cause your bike's clutch to slip.

 

Thanks Zenmervolt, That does help. So 20w50 it is. I have some Amsoil Motorcycle oil I will use.
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Can't do better than that. I did my homework when I moved to the US, Amsoil is the winner here.

 

Thanks Zenmervolt, That does help. So 20w50 it is. I have some Amsoil Motorcycle oil I will use.
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Sorry for the re-post how do I get it deleted?