Would Bel Ray Thumper Racing Oil in 20W50 be a good oil to run on a High Compression 686? Or should I run something like 20W40?

 

Have you looked into the Maxima line of oiles? I ran 15w50 in my Rap for a while and the stuff is simply awesome. I could ride a while, step off the machine, and lay my hand on the cylinder. Seriously. It's not convenient for me to pick it up anymore, so I go with another automotive synthetic nowadays. BTW, expect to pay about $10 for a quart. Here is a link...pick here.

 

If you are running a stock rod, you should consider synthetic oil like amsoil or Castrol R1 or something.