If you've converted to a roller cam and are using a high-volume oil pump, have honed out all the oil passages, and basically built it for very high volume oil flow, then maybe it would be OK.
If you still have a flat tappet cam and haven't blueprinted the oiling system for extremely high flow rates, then I'd stick with an HDEO in Xw30 or Xw40 grade. I personally run Shell Rotella T6 5w40 synthetic in my 60s vintage Mopar 440s. I originally intended to use 5w30 in the '66 when I built it and went with an HV oil pump. If I had it to do again, I'd reduce the pump flow rate since the pressure consistently runs 70-80 PSI (hot, above idle) with the 5w40 oil. I'd rather see it in the 60-70 range, which is what the stock engine in the '69 does on the same oil.
Its not so much the viscosity of modern 5w20s and 0w20s that bothers me in a vintage engine, its the additives. All the Xw20 oils are fully intended for *modern* engines with roller cam followers, light valve spring pressures, roller timing chains, and no distributor/oil pump drive gears at all, so they've got lower doses of the high contact pressure additives (ZDDP and its replacements) than heavy-duty engine oils like Delvac, Rotella, etc. If RT6 came in a 5w20 grade, I'd probably try a run of it and seen how the UOA came back.
