Thanks Rag, I am very pleased that is looks effective to someone besides myself! I would be happy to share my method with you. I used Testors Model Master enamels to do it. I sprayed on an initial coat of Radome Tan for my base coat of CDL.

I then added a few drops to that mix and randomly sprayed patches on the wings to prevent them from looking to monochromatic. Next I sprayed some more random spots/patches with Dunkelgelb, which is sort of a dark sand. This further blotched my finish, which was exactly what I wanted.

I then applied the thin strips of tape to mask off the rib areas and sprayed a little dunkelgelb over them.

To keep this from appearing too stark of a contrast I sprayed a little dunkelgelb on the entire wing surface after removing the tape.
This tied everything together.

For the oil spots I used a brownish mix and sprayed a puddle on the wing (I have been spraying at about 15 psi or so this whole time. I now turned the pressure up to at least 20 psi and sprayed that puddle around with just air (no paint) I was using a double action brush for this. I just 'moved' the puddle around with my airbrush just as the slipstream would do on the real plane until I was satisfied with its look. I also did the same thing with Burnt umber.
Hope this helps,
Chris