Thank you, this is the info I was looking for.
The back story is I was working on a bike (5-speed Evo Harley) that the owner had put BelRay gear lube in. I've seen pretty impressive results from BelRay in terms of quieting things down, but this bike has an intermittently leaking main drive gear seal which is not going to be changed any time soon and the tacky/sticky gear lube was making quite a mess, which is especially rough on a rear drive belt.
That stuff doesn't wash off very easily at all. Not off bikes, not off clothing, it is seriously messy stuff.
Motor oil was originally spec'd for these transmissions, and I wondered what would happen if I put a straight-weight oil in this transmission. If nothing else it will be easier to clean the oil off the bike.
All I had on the shelf was 20w-50 Mobil1, 15-w40 Rotella, 10w-40 Mobil1, and some aftermarket SAE60 formulated for Harleys. I decided to try the SAE60 just for an experiment.
The shifting is a little louder clunk but smoother and more positive. The gear whine that was there with the BelRay is gone. The operating temperature of the transmission is about 125 to 150 when the head temp is about 220 after a 20 mile test ride.
And it completely quit leaking.