You can get oil from NAPA that is made for older engines with zinc. I can't recall what it is, but I put it in my 1930 Marquette...It's on the shelf at my local NAPA, but I know it is made for pre-1960 engines...more zinc, etc. The engine in my '30 was rebuilt in the early '70's, so I don't take any chances...it runs like a top and holds good pressure (40 psi).
I use the pre-SM rated oil in my 1972 cj5. Just my own piece of mind, since the 304 is the original from 1972. I've had the cam and main bearings replaced, but that is about it. Not a full rebuild as it was clean.
As for the lead additive, I use it in my '30 every other tank. I don't drive it more than 60 miles a year, so it won't make a major difference. Only thing is that the ethanol in fuel today ruins old school mechanical fuel pumps and diaphragms. Had a tech notice from Buick on this matter...new fuel wreaks havoc on the older iron in more than one way...lots of corrosion and rust from the fuel condensing...
Here is a nice rating chart for years of oil and vehicleshttp://www.upmpg.com/motor_oil_rate_gas.htm