Never use less than 10w40 oil in the SOHC4 CB750 engine: Honda recommended (and I have found this 100% valid) using 20w50 oil unless you ride in temps less than 30 degrees, then use 10w40 or 15w40.
It was a mis-translation of a Jinglish Owner's Manual in 1974 that started the whole 10w40 oil thing in these engines. When I go inside them, I can immediately tell if 10w40 oils were used: the crank bearings are worn more and the clutch plates show slick, burnt spots on their faces. Eventually this lets them slip.
While I changed the plates on my 750K2 at 55,000 miles (after I quit roadracing it, and got married), I merely measured them at the recent 138,000 mile teardown I did in 2013. I rebuilt the rest of the engine then, but put the clutch back in, as it had less than 5% thickness wear since 1980 when I did it the first time. I run ONLY 20w50 oils (with lots of zinc), and anyone who has ridden with me on dry roads can tell you I am not nice to my clutch!
