First off, I would never use straight weight oil in anything I own unless it was a prewar classic that has never had its engine rebuilt and has had nothing else but straight weight oil in it.
I use either 20w -50 (any brand) or 15w-40 Shell Rotella T4 in all of my cars that have their original engines that haven't been rebuilt but are "worn in". (Shell Rotella has more zinc than most conventional motor oil but they've cut back somewhat to about 1200 ppm down from 1400ppm)
If you are concerned about the lack of zinc in modern oils, I would use Classic Car Motor Oil made exclusively for the Classic Car Club of America. It is made specifically for antique cars and has the most ZDDP content of any oil available - 1600 ppm.
I would never use a zinc additive in any oil for many reasons, one of which is that there is no way to ensure a uniform mixture of the additive throughout the entire crankcase oil volume. What do you do when oil has to be added?
Anyway go with the CCCA motor oil if zinc is a concern for you.
Personally, I have never been concerned about the zinc content in any oil. I've been driving my '56 Cadillac for 24 years as my primary vehicle and have never had a problem using regular oil - either 15w-40 or 20w-50.
www.inrccca.org
