Another way to apply oils is to dab the oil paste on the car where you want a color effect.  I use a very small brush, a large paper clip, or the blunt side of an Exact knife blade.

Drag your brush through the oil to fade the color down the car side.  If you are doing the roof on the auto carrier below, stroke to both sides.

Brush wetness depends on amount of oil, size of area being colored, and depth of effect wanted.  Look at prototype photos.

For instance, gravity pulls a rust stain down the side of a car - vertically.  It is more intense at the top and less so as it moves towards the bottom.

The great thing about oils is their working time.  Too heavy - blot the brush, re-wet in thinner and stroke the car which reduces the effect.  Or just wipe it off with a thinner dampened cloth and start over.  Again, use multiple coats till you get the result you want.

This is the re-worked car with a more subtle effect.  The turpentine brush strokes also provided a fade effect to the base color. The effect with this process have a degree of transparency.