Perhaps off topic... but since flat tappets were mentioned...

If you've got flat tappets, be sure to use ZDDP additive in your oil, or use oil with zinc additive already in it.
All modern engines now use roller tappets. It is so common now that almost all of the oils you see for sale in parts stores do NOT have the kind of friction protection needed for flat tappets! Oil that is appropriate for a flat tappet engine is now hard to find!