The term flat tappet is normally used in the context of pushrod OHV valvetrains. There are not many of these OHV engines still being produced in today's cars. An exception is the GM V8 used in larger Chev cars and the Corvette. But for about 20 years now, versions of that engine have used roller tappets (aka cam followers).

The pushrod/OHV setup puts a high loading on the cam-to-tappet interface, especially in the case of flat tappets where the contact is sliding rather than rolling. Hence the desirability for an anti-scuffing (zinc-based compounds) additive in oil. The latest specs (API "SN") reduce the zinc component to reduce catalytic converter poisoning.

I would say that us OHC B21/23/230 owners need not be concerned. The loading on the cam-to-shim/bucket interface is not anywhere near as high as on a typical pushrod engine. The Volvo cam lobes are quite wide, and valve springs are relatively light. Nearly everything on the market for many years now is OHC and the makers have no problem with these latest oil specs.

Just use good quality oil and change it according to your owners manual.
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Bob: Son's XC70, daughter's 940, my 83 240, 89 745 (V8) and S90. Also '77 MGB and some old motorcycles