API SM grade oils are tested to be used in all motors, and the test protocol includes testing in flat tappet motors. They mean, all motors.
Read the specs and understand them, you'll find API is saying SM grade low
zinc, low phosphorous
oil is OK. They are putting their certification process on the line and their reputation. They did that because - they test it in flat tappet motors.
If that's not enough, they also recommend you can use a diesel grade 10-30.
Zinc and oils for older engines
It really is an old subject, and much ado about very conflicting stories without analysis and back up. Again, the use of imported iron lifters during a sudden shortage happened at the same time - yes, lifters failed. What we know as a fact about that should be verified by actual tests from reputable sources.
I should go without saying that if their tests aren't good enough, then why trust the rating at all? Nonetheless, they still test in flat tappet motors, to certify the
oil for
all engines.
I didn't completely understand that an hour ago, but now I can at least address it having read the specification. I must not be that old a dog after all.