Oil Weight

Since the OP mentions flat tappet, then he needs very much to learn about ZDDP and ZDDP additives.

Most of the lighter weight range oil are now too low on ZDDP for reliable flat tappet operation, around 800 ppm. These are the 5W-20's and 10W30's, and this applies to many/most of the newer SN rated oils. It has been well established now that you need 1200-1400 ppm for flat tappets to live reliably with heavier valve springs and non-stock cams; your stock engine may survive with lower ZDDP pressures on low pressure stock valve springs and cam; the were lots of older engine that lived a long time with stock springs and the older low ZDDP oils.

Once you get higher in weights then the limits on ZDDP set by the EPA are relaxed, and you find the higher ZDDP levels more commonly in them.

Post #3 here shows 2 examples of an oil with the adequate ZDDP (VR-1) and one with additives put in by the user (QS 10W30 + ZDDP additive).

There are specific ZDDP additives and products like Rislone (1/2 can per oil change) can be used too. You have to be careful with ZDDP additives and not overdo it; too much may cause problems like surface spalling and ZDDP will fight the detergents in the oil.

Synthetics can have high or low ZDDP levels just like non-synthetics. Synthetics will lower bearing, ring, and piston wear dramatically. I have used them for 40 years from /6's to hard racing, and cannot say they nearly as much to lower lifter wear, and hydraulic lifters seem to me to be noisier with synthetics. Synthetics oxidize at a far low rate than non-synthetics so will last much longer under hard use.