Don't forget, the SAE rating system doesn't care if a fluid is synthetic or derived from dead dinosaurs, the viscosity is measured the same way. And Harley has been using 50 weight in the primary for a fairly long time, so I don't see that it would cause any problems as long as the fluid doesn't contain friction modifiers or other stuff that isn't compatible with a wet clutch.
I believe the Redline MTL is more like a 10w40, but that's still not what I would call "thin". I personally use and recommend ATF in the primary, btw. I'm not convinced that the lighter weight fluids "create" more noise, but even if they did I believe the decision on fluids should be made based on mechanical performance, not sound. And ATF definitely works better with my Harley clutch.
Jerry