My extensive experience in the paint industry was long ago, but I doubt things have changed that much. Epoxy paint usually contains ring structures in addition to polar bonds and crosslinks. Like dissolves, or swells like. The esters and soaps in the synthetic additives might be more aggressive towards the epoxy, but I would be very surprised if even concentrated they attacked epoxy let alone at levels in a complete oil.
Antifreeze and brake fluid are another matter. For all you brake fluid changing fanatics, save the old fluid, it makes a cheap, effective paint striper. You can dunk small objects in it and let them set a few hours, and wash it and all the paint away. Brake fluid will attack all but the best epoxies and urethanes. Do not spill it on your car's paint. If you do, wipe it off and wash with soap and water immediately.
Paint inside gearboxes? I have to admit I haven't seen inside many gearboxes but wouldn't expect them to be painted. There is a current thread here discussing synthetics and seals. You might be more concerned with the effect on the seals and maybe syncro rings than on the paint.
