So is calcium one of the ones that can be bad for the cat-converters or is it bad in a GDI engine for other reasons? What about Moly? Is it one to avoid also with GDIs? I know some oils tout the fact "high Moly."
Calcium is only a detriment to certain GDI engines (more so TGDI) in terms of LSPI. It's just a bit too reactive. The sulfur can be harmful to cats, but it would take quite a bit to have an effect and the same would apply to any other additive containing sulfur.
Calcium sulfonate's root is a calcium carbonate salt (chalk). Take some chalk and scuff a bunch of it in a small area on the sidewalk, porch, or wherever. Take a glass, or something with a flat bottom to it, and slide it across the deposited chalk layer. You'll notice how the chalk fills in the gaps of the surface and forms a rather slick layer. This is weak compared to the MoS2 deposited by MoDTC, little more than enough to cancel our ZDDP's friction increase, but it's still something. The downside is, since that CaCO3 deposit is still basic, it can neutralize ZDDP's acidic nature in high concentrations thus ZDDP increasing helps counter that to maintain a balance. ZDDP should always be the dominant additive.