I have had problems using oil base paints (Floquil) for the MTH figures. The paint sometimes reacts with the plastic and NEVER dries. You end up with a sticky surface.
Unless you were using different kind of Floquil paints,. they weren't "oil based." Standard Floquil Paints were Lacquer based. The lacquer would have attacked bare plastic. On figures it would have, well, disfigured them. Floquil also made a line of paints called "Polly S." Those were water based.
I use Tamiya alcohol based acrylic model paint for figures, and most railroad painting in general. It goes on smoothly, covers well, dries quickly, and come in a wide variety of colors, including the flat colors you'll need for figures. The N-scale passengers waiting in the station were all painted with Tamiya paint. By hand, by me. Tedious, sometimes frustrating, but rewarding.