I just called Amsoil product support to ask that question because I also wanted to know , he said diesel engs make alot more internal chemical contamination and moisture buildup do to the high compression of the less refined diesel fuel and alot more moisture buildup inside diesel engs therefore the additives that are put into diesel oil need to be different than that of gas . Now I guess we know !That is a good read. So why dont we run a good gas oil in our diesels because they are far superior to the diesel oils?
1200ppm isn't much. Cenpeco oils have more than that (approx .1% zinc=1000ppm). Cenpeco's S3 oil has ~1600ppm (0.16%) and their Super Racing Oil has ~2400ppm (0.24%).
It has been seen on UOAs that confirm those levels. For example, on BITOG (Bob Is The Oil Guy) a guy with a 6.7 powerstroke did a 5k interval on S3 oil and still had ~1400ppm of zinc from a certified lab. Pretty hard to complain there.
When it comes to high zinc/high performance based oils a synthetic can't compete IMO. Brad Penn and Cenpeco are a non-synthetic when it comes to racing oils and use a different base stock. They all start from a paraffin base for higher film strength and better lubricity.
Some interesting reading if you're interested: https://540ratblog.wordpress.com/