Is it absolutely necessary to use a medium?

[Tom Edgerton]

Jean-

Paints are formulated for the best "average" handling, that is, for the way most people paint. Like Michael says, the additional media are to customize it to handle the way you want. If paint had to be heavily altered by everyone with a medium to make it handle a certain way, the manufacturers would just make it that way, generally.

Again, fatter (oilier) paint over leaner layers is a pretty safe bet.

The good news is that oil painting is a pretty athletic, elastic and forgiving medium, that tolerates a lot of abuse and experimentation before it implodes entirely. You can get away with a lot.

The discussion about various oils and recipes gets arcane fast -- everyone has their favorites. Old Holland used to publish terrific technical "teaching" letters that taught me loads about sounder technique, written by a Prof. de Beer. I don't know if they're still around or not (someone sent them to me). Might be worth a web search, when we can get to it.

Happy painting!