Besides the paints of Oil and Grease, is there a way to take the paint I have and make oil and grease?

Yep, Real Grease! Try melting vegetable shortening (Crisco), applying it with a fine brush, and lightly dusting it with black/dark grey chalk pastels. Apply sparingly, however, as too much leaking grease is indicative of seal failure, and would make a tank a pillbox quicker than an APFSDS round. Regards, Rob

clean grease and oil is a golden brown color and as it is used it darkens eventually turning black, so take it from there in color selction. Simulating it on a combat vehicle also take into consideration dust, dirt and sun exposure's effects

I've tried to go for the real one but I find quite difficult to properly control oil when applied to a tank and often the effect can be overdone. Grease is too "instabe" in my view. If the temperature in the room raises or if you photograph it with artificial ligths it will melt or liquify... so I prefer to paint oil/grease stains.
Ciao

my prefered method is to paint stains onto vehicles, i use oil paints, for grease stains use only slightly thinned paint and apply sparingly to your model in areas where grease collects on real afvs ( wheel hubs etc ), then add dust according to how long your afv has been in service, pastels are best for this. for oil stains use heavily thinned paint and when painted on use a hairdryer to dry it quickly, when done right this gives a very realistic effect. be sure to get two different colours to simulate oil and grease. happy modelling!

Guess I will just buy the oil and grease paints. Sounds the easiest.