[Gearbox]

I agree, most likely you have some pin holes rusted through the tubes inside your heat exchanger or your getting oil dripping in. If it's an oil smell, check for leaks. If it's a CO2 smell. Either take them out, carefully cut off the heat exchanger shrouds and examine and repair, or replace the heat exchangers, or forego heat. Not a good thing to have CO2 leaking in the cockpit. I suggest keeping the windows cracked open until you resolve the problem, Good luck Allan