I have recently come across a great product, which when added to the current 'SM' oils, brings the oil to the 'SF' rating for pre-1988 flat-tappet engines. The product is ZDDPlus. After much research, I have added it to all of my '69 Camaros. In fact, I am so confident in the product that I am distributing the product. If you are in need, feel free to PM me.
ZDDPlus
Check out the website especially the tabs on 'diesel oil' and 'racing oil'. Great information depending on the usage of your vintage ride.