I'm not sure.
I just push the "paint" around until it looks right. I'll usually start off with exaggerated base colors and then smudge them so that they're more blended and subtle. Sometimes I do it with a low transparency brush too. It depends on my mood.
Is there any way you could simulate oils with whatever program you're using, maybe? Have you ever tried Artrage? It simulates traditional media a bit better than Photoshop or Painter.