Hi Tom

Ford originally specified 20W40 for the 351C.  If this is a 45 year old engine still running on the standard bore, original rings & bearings, then 20W50 has most likely been used most of the engine's life; or possibly 10W40. An old engine like this is already worn enough & carboned-up enough that it doesn't warrant synthetic based oil. I would also imagine the rod bearings are quite worn, and hot oil pressure runs low. Its supposed to be nominally 60 psi above 2000 rpm.

My recommendation is Valvoline VR1 because of its additive package and because  it is reported to have very high anti-wear properties. Petroleum based, 20W50. If the outdoor temperatures are low, then 10W30 … if the oil pressure is OK operating with the lower viscosity. No additives.

CASTROL GTX HIGH MILEAGE 15W-50

AUS-Hi-Mile-5L-GTX-15W-50.jpg
This is what is in mine with 55,000 miles
GTX HIGH MILEAGE 15W-50 oil provides extra protection for engines with over 100,000 kilometres. As an engine gets older gaskets become brittle, seals can deteriorate and oil can burn faster. Castrol GTX HIGH MILEAGE motor oil helps extend engine life by providing superior protection against deposit build-up

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