This is somewhat related to cbfomoco's post below, but since I have reported on it twice in other forums, thought to do so here.
I was using Valvoline 20/50 in my 292. Recently I replaced the crimped off overflow tube from the rockers, with a stock unit. Afterward, started the motor with rocker cover off, and waited and waited. No flow to the topend.
Alarmed, I shut it off after about 1 minute. Temp outside was 59 degrees and the truck was probably 50 degrees. Thought about it, and could think of nothing I had done to stop the oil. Started again and about 10-15 seconds later oil started flowing all over the place. The oil did have an additive, SciTech brand.
If you are running 20/50, my opinion and experience is it is not a good idea unless your motor is at the end of its usefulness. Also, the guys in the Oil and Lubrication forum, indicate that SciTech additive is a bad idea. Too little ZDDP and too many other bad side effects.