You should not be breathing anything at all. I have an electronic CO detector and it reads zero during normal ops.
CO doesn't stink
Every time I was on a single engine
turbine aircraft, I noticed strong exhaust smell in the cabin, especially during ground operations. Turbine exhaust is oily and has a much stronger smell than petrol engine exhaust. In a multi engine turbine, you are not seated directly behind the turbines so it's less of an issue.