I'll agree on the additives- oils already have a robust additives package and adding stuff to the oil messes with their formula. Seafoam is normally a fuel additive and most don't use it in the oil, but some do run sea foam in the oil just prior to an oil change to help loosen deposits in the crankcase.
Now you're saying the motorcycle oils don't break down as fast as the auto oil. Any way to back that claim? That has been the hype coming out of dealerships for years.
Here's Gadget's links on the subject:
http://www.gadgetjq.com/Oil_Articles.htm