Yes Thinwater they are closely related. Motorcycle racing motors stress the oils much more than an outboard does. I don't think I made comparisons between outboard and motorcycle engines but you can rest assured that the stress level is much higher in motorcycle engines! (both 2 & 4 stroke) I have used and worked on outboard engines just as long as I have worked on motorcycle engines and that is a long time! Chief (small engine certified)
PS: What may be overlooked here is the fact that virtually all sizes of motorcycle engines have been water cooled for many years.
Again, yes and no. Air cooled engines run at higher temperatures and thus use different additives. Corrosion issues are different. I respect your maintenance experience, but I used to work on the oil formulation side. They are both difficult applications, but they are slightly different. TC-W3 was developed because some air-cooled 2-stroke oils were causing plug fouling and ring sticking. Lower temperature and fixed RPM were considered to be factors. Thus TC-W# oils, for example, are ashless.
Bikes are formulated to TC, of which TC-W3 is a subset. The truth is that most good oils will work fine, and that not all brands go to the trouble of even trying to meet marine standards for a tiny market.
There are also different specs for 4-strokes (auto an marine). Again, many good oils have never tried to certify in this tiny market. I've long used non-FC-W oils, but I sure wouldn't give that as advice. So long as we are discussing this, and if people want the whole story, 4-strokes should meet FC-W. Go to the NMMA site and read up. The specs are there.