Old and wise - oil analysis done years ago indicated that HD primary fluid was just
plain ole 10w40 motor oil; the clutch likes it a lot. Diesel rated stuff is just tougher.
IMHO.

 

Typically this is an indication of proper fluid amount versus the "kind" of fluid used...

 

For My 2cents I agree with Clutchglass and have used Mobil1 in my boat (2engines) car, truck, sterndrives and have never had a problem due to failure . The 75/90 synthetic in trans is quieter and the engine never overheats in traffic .I would stick with what I know works .

 

amsoil says you can use its 20-50 in all 3 wholes(2 if you got a sporty). i used it in my sporty and noticed the clutch seemed to slip with it in.. in the dresser i had some oil leakes do to the idiot who installed my 95" kit so i stuck with regular oil.. still dealing with his crappy work but i use amsoil 75-120 in the trans and hd trans plus in the primary.... people also use amsoil universal trans fluid in the primary too... it synthetic lubs and keeps it all cool.

 

what's the worst thing that could happen using amsoil synthetic in the transmission?

 

Amsoil 20/50W in the motor and primary and Amsoil 75/90 Severe gear in the trans.

KISS

.

 

08 Road King Classic. Have run SYN3 all three holes and dumped it after 2k miles. Engine was very noisy and had constant 5th gear whine. Put Amsoil 20w-50 (MCV) in Engine and Primary and 90w-110 (SVT) in the Tranny. Engine runs a lot quieter, no more 5th gear whine, and shifts into Neutral easier...no complaints. Amsoil recommends the 20w-50 in all three holes to conform to HD recommendations.

 

Ok just for the heck of it. Would you put Gear Lube in your engine? I think not.
Then why would you put engine oil in a gear box?
Just because you can get away with something does not make it a good idea.
years ago we would put saw dust in gear boxes and rear axles to stop leaks and quiet them down. Worked for awhile
All my HD's 75/140 Gear lube and now days both of mine run 75/140 GL5 full syn Spectro

 

In a recent article in Rider Mag, Chris Sidah addressed this issue, he pointed out that the sulfur content in gear oil isn't necessarily friendly to the bronze bushings in older motorcycle gearboxes... or older designs of motorcycle gearboxes..

 

The only "gear oil" that I can find at the base where I'm located is a Quaker State 80W-90 GL-5 for limited slip and conventional differentials. Do you think this will be safe for a 2009 Street Glide? Royal Purple 20W-50 and 10W40 motorcycle oil is available too along with Mobile 1 auto engine oil.