Originally Posted By: CCI
Just re-read your post, I don't think I gave a very good answer last time.
Three concerns:
Foremost is that I cannot kick this bike in cold weather with straight 50 or 60.
Second is the multi-viscosity oils just don't hold up. I don't know why, but it just turns thin in no time and the motor sounds terrible. You can feel the difference at over 80 degrees ambient.
The motor is old, everything that has not been rebuilt is worn out, to the point where I can't run hydraulic lifters anymore because they collapse at idle. Oil pump hasn't produced more than 12 psi hot in decades, probably about 3 psi at an idle.
Third is leakage, oddly enough the thicker oils leak worse when the motor is cold (more pressure I suppose), but once it is hot, the multi-vis oils come out faster.
So today I drained out the SAE60 that had about a thousand miles on it and looked awful and put in 2 qts 20w-50 and 2 qts SAE50. The motor sounds much better on this, we'll see what a little time and a few miles do.
As for effective viscosity at temperature, I know it's supposed to work that way, but over about 80 degrees ambient on any multi-vis oil the motor sounds like somebody threw the entire middle school marching band down the stairs. On straight 60 it sounds great. I have no idea why.
Yeh. I believe straight weights have a lot going for them.
You MIGHT find a mix of 60 and 20W50 will be better and still allow you to kick start. You could perhaps gradually evolve in that direction by topping up with straight 60 (I'm assuming you'll still have enough leakage to require fairly frequent topups) and seeing how it goes.
I ended up with my mix by topping up Delvac MX 15W40 with local SAE40, then getting a massive rocker cover leak. I probably could, and perhaps should, just run the straight 40 but I'm not sure of its quality used alone.