^No one can truly say specifically, IMO, but one oil change interval with MMO isn't going to break your seals or gaskets. It's not super aggressive and will only get things 'moving' regarding cleansing.
You are using a HM oil already, it probably will have no effect at all as far as causing more oil consumption(by adding MMO), unless somehow during the OCI it cleaned out things and 'revealed' a leak. It won't create one in of itself. If the new rear main was going to go by adding a little MMO, it was a lousy replacement rear main anyway, quite frankly. It won't break new gaskets/seals/rubber etc; by itself in moderate use. Occasional is one thing, I've never used it constantly and I don't think anyone on here would recommend using it for long periods of time or every oil change, if that's what you were wondering at all, too.
If it were me, I'd just put Rotella T5 in there during this winter and see how things go, it's an HDEO oil and once the hot weather is gone I doubt you'll need the 40 weight in the winter; unless your vehicle consumes quite a bit already or it calls for 40 exclusively. Either way, the HDEO would be almost as thick as the Max Life 10w40, since it's a HDEO and not a PCMO( a thick 10w-30).
That + MMO during this winter would be ideal for any attempts to 'clean' things up. Question, do you know if you have sludge or just a good amount of carbon deposits...the cleanliness of the engine at all(since the rear main was replaced as well as the oil pump)?
