I did some reach and for flat tappet cams engines you need to be concerned with zinc content. Lots of threads on ZDDP content requirement.
Personally I change the oil often enough that synthetic is just more expensive with little if any value when you consider the extra leaks you will have.
ZDDP is the issue you need to be more concerned about. But if you are going to go Synthetic this was put out in 2012 by Mobil
"For older, flat tappet engines where wear may be more
of a concern, we offer a number of synthetic oils which are higher in
phosphorus than API SN/ILSAC GF-5 oils. These include Mobil 1 15W-50
(1,200 ppm), Mobil 1 0W-40 (1,000 ppm) and Mobil 1 High Mileage 5W-30, 10W-30
and 10W-40 (900 ppm). For all newer engines and flat tappet engines in normal
service, API SN/ISLAC GF-5 oils are preferred for better fuel economy and for
the protection of catalytic systems with lower phosphorous (800 ppm)."
Personally I would not run any oil with less that 1,000 ppm & prefer 1,200 ppm but stay below 1,600 ppm as you are getting in the range that is considered to high and will do damage. I do not like to use additives as I cannot determine what the actual ZDDP ppm is.
And yes I know that GM now says that 800 ppm of ZDDP is good enough with the other oil additives that you have in most modern oils. I also know that the IRS says they are not political because that would be against the law, the Easter bunny is dead, etc.
