I am not an experienced mechanic, so I have the Moyer Marine Manual for the Atomic 4 and I blindly follow it
For additional details not covered in the manual, I heed the advice of the experts on the Moyer forum. There you will find a wealth of info on Marvel Mystery Oil and engine oil.
I adhere to the prevailing wisdom by using Shell Rotella 15W40 Diesel Oil. Yes, diesel. Check out the Moyer forum to get the complete lowdown on the Rotella - more details than I can relay to you but has to do with the changing properties of engine oil over the past 40+ years.
For MMO, yes I use it. Don Moyer recommends use something, so I use it. Don says we should use MMO at winter layup, has worked great for me. I don't know the specifics of where to find MMO in Canada but I don't recall this being an issue for others on the Moyer forum.
As for additives, the Moyer community is largely down on fuel additives, with MMO being the lone exception if you have persistent engine issues like sticky valves.
If you don't know what chemistry experiments your Previous Owner ran in your fuel tank, then consider pumping out your fuel tank and starting with fresh gas, ideally ethanol free gas if it is available in your area. That is what I did, pumped out the fuel tank and then added it to my truck - no problem.
C&C 27 Mk I, #45, Vermonstah