I finally gave up on my old standby, Quaker State oil, after I noticed varnish and gunge appearing inside the valve cover, on the dipstick, etc.
I decided to switch to Havoline to give it a try. Since I had a little room in the crankcase, I put in about a pint of Mystery Marvel Oil before pulling out for the Labor Day weekend.
I was not able to make any scientific measurements, but the engine seemed to be quieter.
When I drained the 3000 mile Quaker State, it was blacker than the hinges of hell.
Got a chance to look inside the valve cover, and it looked clean.
This is no definitive endorsement, but seemed to be a harmless experiment with reasonable results. I don't intend to run the Mystery Marvel oil as a regular treatment.
Now, I hope the Havoline will run as clean as the old Quaker State did.
If not, will keep on testing.