What's the drawback of over-oiling?
Where to start?
Basically, over-oiling attracts dirt and grit particles to the working parts of your locomotive, making a very effective grinding paste that will make round holes into oval holes. If oval holes were what were needed, then they would have been made that way in the first place.
This is my Accucraft NGG16 Beyer-Garratt, bought in 2007 and a very fine and powerful performer. In spite of oiling as carefully and sparingly as I can, it recently cost me over £700 to have all new bearings and bushes replaced.
Instructions for ANY mechanism that requires an added lubricant will ALWAYS have a warning, something to the effect - 'DO NOT OVER-OIL! OVER-OILING CAN BE AS DAMAGING AS NONE AT ALL! USE A SPARING APPLICATION OF LIGHT MACHINE OIL!' Or similar.
Do you actually OWN a steam locomotive yet? If not, then the instructions that come with it will advise the same thing.
However, all we can do is advise you - what YOU do with your loco in entirely up to you.