hmmm... I may have to do a real Wildcat one day... it doesn't look all that bad in blue either!
Bob- On the Martlet I used a mix of Paynes grey and burnt umber, but on your you might want to stick with a black or very dark grey. Just get some of that turpenoid from the hobby lobby or whatever is near and mix it just like an acrylic wash. On a completely futured fuselage just take a soft wide brush and brush an entire section at a time as in a whole upper wing. You can immediately take a paper towel and wipe off the excess in the direction of air flow. Then get a fresh paper towel and begin wiping off the oil until it looks just right to a little heavy. Any deep crevices or detail near a part junction (wing root) can collect a lot of oil, use a pointed end brush to drag this out till you can get at it with a qtip or a piece of paper towel. Just work at it like this till you like it!!!
On the lower wing surfaces it works particularly well as the counterweights will provide a streaking effect after them just like you see on the real things... the various protuberances will do the same thing...
After you are happy with it let it sit about 2 days before coating it as it will still be somewhat workable. This is a pain BUT the thing is if you decide something looks a bit off you can still correct it in the first day or so after washing... oil stains can be done the same way by adding a litle burnt umber to the mix, placing a dab along the panel you want it to be coming from then use a paper towel to LIGHTLY run it back in the direction of drip or air flow...
if it looks overdone after drying you can take some powdered pastels and brush over the area to lighten it... I did that on the wingtops and upper fuselage as well to simulate some fading... the future washed some of it off when I sprayed it on but it actually provided a somewhat mottled fading which was even nicer...
Good luck!! If that didn't make sense let me know, sometimes I confuse even myself!