A friend recently inherited a Harley Evo. It came with engine oil which he is going to continue using for the engine (He thinks his dad was using engine oil in the engine, primary and trans). He has access to Mobil Delvac 1 Transmission Fluid 50 (MT-1 / GL-4) at work which is a 50 (Engine) or 90 (Gear) Grade MTL with viscosity of 17.5 cSt @ 100C and 146 VI. He was thinking of using it for primary and trans. Any thoughts?
Harley used to call for seperate products in all 3 applications but in recent years has consolidated to only one product for the primary and trans.
The context to keep in mind with all of the different answers you'll get is that the Evo is an incredibly tough engine and the clutch and transmission are every bit as good. Any of the premium-quality oils suitable for use in V-Twins (high detergent/low ash/anti-foam) are going to work fine.
I've ridden an Evo that had a quarter-million miles on it and the only oil that was ever run in it was Rotella 15w-40. The only work that was ever done to this bike was oil changes, spark plugs, and tires.
Of far greater importance to the longevity of the bike is how you ride it. Let it warm up a little in cold weather, use progressive shifting, avoid hard shifts, accelerate and decelerate smoothly, keep the RPM below 4K as much as possible, load the drivetrain before you apply power, and just go generally easy on the bike.
If it feels a little like you're driving a truck with an Eaton Roadranger driveline you're doing it right.
I have no idea how many Harley clutches and transmissions I've been into and I don't see any difference at all in the service life of either as long as an approved lubricant is being used.
All that said, I would agree 100% with alarmguy:
"If he wants to do what is best for the bike in my opinion is just use the called for lubricant which is still Harley Davidson Transmission and Primary Lubricant. If not I think its a bit of a wild card but doubt it would hurt the transmission even if its not the purpose that Mobil states."
I'm impressed with the way the HD Transmission and Primary Lubricant works in Evos.
As for the engine, any of the 20w-50 oils specifically designed for V-Twins is going to be fine. I noticed you wrote "inherited" --If you run M1 you may see a lot of crud coming loose if the bike has been sitting any length of time.
And perhaps most important of all -- tell your friend to clean the tappet oil screen right now, and again at the next oil change, and every oil change thereafter. It's under a cap at the top of the right engine case casting just forward of the oil pump. This is really important to do.