Sorry, I should have responded to this - I am running 20W50 Valvoline VR1 mineral
oil. Its winter here, so temperatures are zero to 10 degrees C (~32-50 Farenheit) when I get to the garage, so I know that'll make the
oil a bit thicker.
Once the fans click on and the engine is pumping out some heat, it goes down to about 70psi at idle (~1000rpm).
I should have mentioned its a fully re-built 428, needs firm but comfortable effort on the breaker bar to turn over the engine with the plugs out, so compression is good, but I dont know actual CR ratio, but the engine is 'tight'.
Any recommendations? People over this side of the pond all suggest a mineral rather than
synthetic or semi-synthetc
oil for this kind of old engine.
Thinking I should change to 10W40, as I am worried about the strain on the distributor drive on start-up, and weather those big 'W50' molecules are getting into all the places they should.