Try Rembhrant oils mixed with mneral spirits. I use Burnt Umber for oil and grease and Burnt Sienna for rust tones. As you do and read more about modelling, you will end up purchasing these oils for other sues anyway.
Burnt Umber and Sienna were used as washes on this JSU-152. MMP Weathering powders used for the dust and oil washes used to stain the wheels for oil spots.