My engine doesn't like
synthetic so that's out. Brad Penn is good stuff but not readily and easily available.
Solution for my flat tappet old school side oiler? Shell Rotella 15-40 and a can (or two because I use 12 quarts of
oil per change) of STP. Rotella has the ZDDP as does STP, together it apparently is "enough". Coming up on 10,000 HARD miles with my flat tappet and looking good!