I purchased 10 bottles or 10w40 Bradd Penn full race oil, "0" detergency, but the added buffer materials - zinc/phosphorus IIRC.
Not catalytic friendly, and it says so.

They were in the $12 range, a bottle.

I got it from the local Stinson's supply depot here in Ottawa.
Seemed to run fine with my 6 bolt setup and sandwich style oil cooler. Didn't get to put many miles on it though.
Those bottles that are left, are now 3 years old likely.
I'm back on detergency until the car is running for along enough period of time.