Try Redline heavy with shock proof,
Mobil 1 is good also
I am currently running Lucas 75-145 synthetic
The Redline cuts down the clunk
Plus it is designed to cling to the gears better
You can view on the site what a burned up tranny looks like
You will see shifter forks burned blue
Spectro makes a Platinum 6 Speed gear oil
Baker the leader in the Tranny industry ships their trannys with a quart of this lube
I have run this in my 07 Dyna,it was a very good lube
Much like most oil threads you will see many likes
IMO your ride you decide
I just hate seeing motor oil in a gear box