Is it OK to leave my window AC in over the winter with a radiator right underneath it?

I left two units in the windows over my steam rads for three winters in my old place because hauling them down a narrow stairwell solo was not happening. Zero issues. They fired right up each spring and cooled like always. Radiator metal that you can rest a hand on is running way below anything that bothers a compressor or the guts of a cabinet, even after months of warm air drifting past an off unit. The housing is built to sit in 95 degree sun all summer, so rising 150 degree air a foot below it is not a real threat. Your towel trick is the part I'd actually upgrade. A towel breathes and lets humid indoor air creep into the gap, which is where you get condensation drips and cold drafts, plus it does nothing against wind whistling thru the sides. Cut a piece of rigid foam board to fill the gap between the unit and the window frame instead. Cheap, snug, and you can pull it out in five seconds when summer comes back. The cord is the one thing I'd fuss with. Insulation on a power cord handles those temps fine, but I still wouldn't let mine drape on hot metal all season, just because it slowly flattens the jacket and gets stiff. Loop it up and zip tie it to the grille or tuck it behind the unit so it hangs clear. Pulling it out every year mainly costs you the gasket seals and your patience. If the gap is sealed and the cord is off the metal, leave it in.

I dunno if it's the direct heat, but I think the mounting itself is where you'd run into trouble over years. If the radiator is right there, the constant thermal cycling could stress the plastic where it meets the frame, making those seals crispy. You've got those friction points where the unit locks into the window. I've heard people mention getting issues with the rubber gaskets if they're exposed to that kind of heavy winter heat every season. I'd prolly reccomend taking it out just to be safe and keep the fit tight.

I used to live in an old school building where we had these massive steam radiators, and I noticed a specific thing happen with units left up in the winter. The plastic fins whould get a film of mineral dust from the steam every single year. After the heat has been running, I usually clean them with a quick blast from a compressed air can; the debris from those radiators is just heavy and sticky. If the heat is high enought to make you sweat, you're likely past the point where it affects the internal components, but those fine dust particles do need to get off them before you start cooling.

You keep focusing on the unit itself, but if you leave it up for years like this, I'd worry more about the mounting bracket and the window frame integrity. Those old apartments aren't built for constant thermal stress. Even if the plastic holds up, the metal of the bracket expands and contracts a little every single season. Over a decade, that movement puts stress right where the unit locks into the sill. If the original screws are loose, you're going to see the seals failing from vibration and movement rather than direct cooking.