I prefer seafoam.
If you are sure U r changing the engine then no big deal to leave it in I guess especially since that MOBIL1 15w-50 is among teh most stable oils out there.Still just giving it one oil change would really help out the next guy who is buying your engine.
ALSO what if delays come with your new engine, they usualld do regardless of were u order from these days.
Better not abuse the current engine just in case.
Fred..
