Parts washing solvent.
It think I have shared this story here before but when we had the T-28 we were properly motivated to find something to remove oil and exhaust residues. The thing was that all 28s have a round engine that leaks oil like a sieve with exhaust runners that are short stacked and faired close to the fuselage. It baked the oil-exhaust-lead deposits onto the stainless steel exhaust shields with a vengeance. Scrubbing bubbles and brillo pads was the go to within the community for years before somebody mentioned it might not be compatible with aluminum so that sent everybody searching for a replacement. Everything we tried worked worse than the scrubbing bubbles and even that took multiple applications to get down through the baked on residues. One day after a flight I have no idea what made me think to try it but as a test I opened the parts washing tank and dipped a paper towel in it. The caked on baked on exhaust stains came off with one wipe, no effort required. It was a revelation. The only residue left after the solvent wipe was a few streaks that came up with a quick application/buff up of regular old car wax which had the added benefit of making the airplane look good. In fact, once you got far enough away from the exhaust tips, just the plain old car wax did a good job of cleaning the lead staining. I would use solvent from the exhaust ports down the fuselage until about the back seat and then just car wax for the remainder of the empennage.
The stuff we had was mineral based and made by Solvoil. No clue if the newer citrus based solvents would have the same efficacy.