Take a guess about the thick residue on this rim

So it was time to replace the rear tires on my '78 D300 Adventurer dually. It has its original 16.5" Kelsey-Hayes rims, which are in excellent condition and are powder-coated, so of course I wanted to wash them up a bit while I had them off.

(Incidentally, Firestone is the only tire manufacturer that still makes a 16.5"-diameter truck tire in a radial. There are several brands of bias-ply tires in 16.5", though.)

But one of the inside K-H rims had a build-up of about 1/8"-1/4" of a really hard, sticky residue, composed of either axle lube or brake fluid (and of course mixed with dirt) that was extremely difficult to remove. Neither mineral spirits nor strong detergent seemed to affect it very much.

Haven't tried using acetone or lacquer thinner yet. Should I? They won't attack powder coat, will they?

So far I am scraping this stuff off with plastic putty knives, and it's very time-consuming.

Anybody care to guess whether this residue was from leaking brake fluid or from leaking axle lube?

I'll tell you the answer later!