I use Valvoline Racing Synthetic (VR-1) 20W50 in my Triumph TR4 and 1933 Chevrolet. It has a higher level of ZDDP for older flat tappet engines.
I have owned my 1933 Chevrolet for over 40 years and it over heated easily in warmer weather until I switched to the VR-1 synthetic oil. The oil must have reduced the friction enough to eliminate the over heating.
I have also heard ZDDP destroys Cats, so I use Valvoline SynPower (Synthetic) in my 1984 BWM.
I buy all my oil for a descent price from Amazon and get free shipping. No need to leave my house and I get the oil and filters in 2 or 3 days.
Like some of the others, I don't believe in additives either.
Vila
1933 Chevrolet
1962 Triumph TR4
1984 BMW 633 CSi