In answer to the "anyone using Rotella diesel oil... ".

Yes. I have run 15W40 in my BB stroker and other classics. It has (still) a decent amount of zinc and viscosity seems about right for 3 season use in a typical performance engine. Originally these engines were designed for 10W30. With bearing clearances opened up a bit typicaly, a little more viscosity helps maintain hot oil pressure... especially on the street in stop and go traffic.

For most engines, agree 20W50 is probably on the heavy side though your engine will tell you what it likes. I look for 20+ psi hot idle on a fresh engine.