Brad Penn semi works great with flat tappets, it's all I used when I can find it, plenty of zinc and boron. Unfortunately my local supply dried up and not worth paying shipping.
I also like the 15w40. Never ran Joe Gibbs, but likely good. I have run Schaeffer, decent stuff, Valvoline VR-1 is good and easy to find, and still have a few gallons left of early diesel oil