Brand should not matter. The oil should meet the specification required in the vehicle manual.
IMHO - A rebuilt engine should be virtually new, mechanically, at least within the limits of what you did as a rebuilder. The oil spec should be the same as that when the vehicle was new.
Since the engine has been opened, and some contamination (swarf) could exist inside, I'd run the first oil change only briefly, perhaps 1000mi, before changing it and then returning to normal intervals. Thus, I wouldn't bother with a high-end synthetic on my first fill either.
JMHO, of course. Oil is more a "religious" choice than it is a technical one, so long as the basic spec requirements are met.