Having now completed 1000 miles in my Mercury since the rebuild in August, I plan to change the oil.
American oils are not that easy to come by here in England.
I used 20/50 mineral classsic motor oil to do the first 1000 miles.
Whats the best oil. I can possibly get our local US parts man to obtain the right stuff.
By the way, the first 1000 miles passed without incident. 
Picture taken in 1960. Funnily enough it still looks like this! 1959 Mercury Monterey 312 Y Block. NORFOLK, ENGLAND, UK.