It's been a 40 year trip.
Back in the early years Valvoline 20W50 Racing Oil.
Then Mobil 1 15W50.
Then the BITOG legend Castrol green 0W30 until my hoard was depleted.
Lately it's been Mobil 1 0W40.
All these are likely far too good for a Briggs flathead, but they represent what I was using in cars at the time.