I'm really happy so far with my repower from Michigan Motors. Old motor was 1985 5.7 GM (Volvo 260a); replacement motor is a 5.7 315hp new GM long block. With the old, rusty, flat-tappet block needing almost all new components, a re-power made more sense to me. The torque band is much improved, so no regrets here.
Michigan Motors wanted me running T4 15W-40, which does drop a little oil pressure after sustained ~3800 rpm use. The oil temp sensor I added shows 230F for those runs in a short time. 260F for thirty second WOT prop testing. Oil pressure does recover after cool-down periods, so my thoughts are that the pressure is dropping from heating of the oil more than shearing.
After 100 hours, I think I'll try the synthetic VR1 20W-50. It claims to have good anti-foaming protection, but I'm having trouble finding real testing of it.
Curious, has anyone here tried out the Pennzoil or Bass Pro 25W-40 oils?