[Special Coupe Frank]

Oil pans can "rust" from the inside-out.

Acids form in the crankcase of older engines, ESPECIALLY those without thermostats, and those that see short-trip usage...

I have a '29 engine currently torn-down for inspection, and there are holes eaten in the dipper-wells on # 2 and # 3, one of the baffles has the bottom edge eaten away, and the oil-pump spring broke right at the clamp, all due to acid corrosion from the blow-by contaminants and condensation.

The bottom of the oil pan is a rough and jagged as a fresh piece of 80-grit sandpaper. No "red / brown rust", but acid etching.

I discovered all this after scraping the sludge out of the oil pan, then soaking-down with an aggressive caustic cleaner such as Castrol "Super-Clean" or "Purple Power"... remove all the grease and oil and left clean metal behind. It doesn't remove or "convert" rust, so if there was "rust", it would have still been there.