What is the right oil to use on a fresh flat tappet engine AFTER break in oil?

I used the Valvoline VR-1 10W30, when my head was rebuilt, for inline 6 with flat tappets.

Since then, I use 15w40 diesel in summer, and any regular 10w30 in winter.
Castrol Classic. Valvoline VR-1 10W40
VR-1 a good choice but only multi grade choices 10W-30 or 20W-50. I’d go 10W-30. Castrol Classic still only 20W-50. Maybe OK for severe duty. I like Brad Penn/Penn Grade 1 for years. They make 10W-40 if you so inclined. I buy a case at a time from a distributor to get a better deal. Driven makes a copy cat oil which is probably OK too. Don’t like Diesel oil in gas motor. That was what was used before ZDDP oil was widely available. Of course, many will swear by Rotella or Delvac.
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