I use black and brown water color, brushed on then I blow it aft to get the streaked look. I then touch it up with a Q-tip to get the excess.
Good luck,
Bud
I use Testors Rubber (a dark brown paint in the small square bottles) mixed with Testors glosscoat and thinned to let it run all over the engine and drip wherever... just be sure to apply this over an acrylic clear coat so it does not eat the paint underneath. This color can be used for leather jackets right out of the bottle since it is a dark brown and does not look very much like rubber to me.
Dave