I used to use Mobil 1 15/50 VR synth oil in my ZJ with a modded 4.Slow, and it worked well, Rotella works pretty good, too as does Delo. I currently use Delo 5-40 synthetic on my 2.5 because of the cold winter mornings. The only thing to watch for if you're being really careful is zinc content. Our engines were designed in the 70's (I think) when oil had more lubricity additives.
As far as RK fluids being good?
I bet they're made by one of the few major oil manufacturers and given a generic additive package, which is to say, they're probably a damned sight better than most fluids that were around when our trucks were new and probably on par with most conventional oil out there now.