I'd say if it was a British plane, add all of the oil that you want

Luft iron, however didn't leak oil too often, if at all, from lack of maintenance.
Like Xasthur said, mud is more the norm. Especially if it was a late war bird. Bomb craters on forward airfields made for some knee-deep slop at times. This, however would dry fairly quickly and would look dusty and smeared.
In addition, if it IS a late war plane, outlines only on the bottom crosses (dwindling resources towards the end of the war, including paint).
Otherwise looks like you are making headway. Don't give up

The more it gets "picked at" the better you will be able to make it look.