replaced valve stem oil seals ... Still smoking ! 273 HP

A good rule of thumb is "smokes going up hill, rings-smokes going downhill, valve seals.

And yes the Marvel Mystery Oil is OK to use.

If it continues to smoke and uses oil, put some MMO in the pan and run it for a little bit then change the oil and filter.

If it's been sitting for a long time it could very well be stuck rings.