Nice work James-
Ref castor oil stains....I used AK Interactive Fuel Stains (AK 025) on the lower wing of my WNW Snipe.

I intend to airbrush some more on the underside of the cowling if I ever manage to complete the rigging.
Be aware too that the Aldis sight and windscreen are tricky to fit. I tried to be clever by mounting the sight on the top wing with the windscreen hanging loosely until the top wing was attached to the struts.
In the event it was a nightmare trying to align the windscreen into it's slot on the fuselage cowling. I would have been better fixing the windscreem first, then attaching the wing and then the sight in that order. Still not much space to work with between the upper wing and the fuselage though.
Good luck with the rest of the build!
DiD