How do you make believable oil stains?

[Dainis]

I've had some success applying drops of thinned enamel on a gloss/semigloss surface and pushing it around with an air from an airbrush to simulate gas/oil stains. But I agree that the best finish is over a matt finish...and the crossing of fingers that you hope it works out. Weathering is truly the dark art of modelling 

[Citadelgrad]

I came to LSP via a return to modeling vie 1/35 armor.  There, the reason for oil paints and even washes is to stain the undercoat. I have never sealed a model before pin or panel washing, as they call it there.  When you arent struggling to et adhesion to a clear coat, its very easy to replicate oil and gasoline stains.