I contacted redline and asked about suitability of their performance engine oil for a Datsun L28. I specifically asked about the 20W-50, their reply...
"Thank you for contacting Red Line Oil, in your Datsun we would recomme3nd allowing the rings to seat with a petroleum oil before switching over to our product, we do sell the Engine Oil Break In Additive that can be used to fortify and increase the phosporus level for cam protection. Once the rings have seated the 20W50 could be used though likely higher viscsoity than ideal, if internal clearances on rods and mains is reasonable the 10W30 would be recommended"
10W-30 seems pretty thin to me?