what oil is safe?

If you read any SAE technical articles by either the oil manufacturers or car manufacturers SM rated oil is more than adequate for engines with stock or close to stock cams and springs. It's when you start going to real radical lift cams with high spring pressures that the 800 ppm of ZDDP in an SM rated oil starts to be marginal.

Those articles will also say it's a good idea to use a supplement with any flat tappet cam during break in.

Diesel oil is not necessarily any better. you have to look at the ratings on the bottle. Diesel oil with a CJ rating has reduced levels of ZDDP as does SM rated gasoline engine oil. Any oil with the highest rating of SL or CI should be good, being synthetic is not the answer either, it's still the serive rating that defines how much of the ZDDP that is necessary to protect a flat tappet cam.

I have always used Mobil 1 oil in my turbo cars because of it's higher temperature ratings and also use it in the Barracuda that has a flat tappet cam because it is more resistant to the formation of acids and it allows me to do one oil change per year. Since the whole issue of ZDDP levels in oil became a hot topic i started putting in a bottle of STP with each oil change in the Barracuda. Anyway, 20k miles and counting on the Comp XE268H cam.