Dan,
I live in the Netherlands. My SPF has a 351W stroked to 427 cu (7lt). I use Valvoline VR1 20W50. Cold
oil pressure is about 65 psi and hot about 62 psi. This is racing
oil and it has more ZDDP as normal, which is good. Not for modern cars with a catalytic convertor in the exhaust. Just for old cars or racing cars. Sold for prices around 50 euro's for 5 lt Not cheap, but good stuff). My engine takes about 5 lt.
Do a search on the internet and you will probably find a dealer that can send it to you. I buy 10 lt at a time, which saves some shipping costs and I have some spare
oil.
Good luck.
BTW It normally is good practice to stick with the oil the engine builder recomments.