I had this years ago on my previous Bravo HGT. It turned out to be a gearbox leak, and the smell of gearbox oil was being drawn through into the cabin.
That said, I'd expect you to be able to smell this all the time, even on air coming in, and not just hot air.
It could just be a little oil has somehow been spilt down the back of the engine bay, and dripped on the heater matrix. Or it could be that it's just not been used for a long time, and the smell is the road fumes, etc heating off the matrix and causing the smell.
Check your water level, and oil level. If all okay, I'd suggest leaving it to run the warm heating for a few days, and it should subside.
If you've lost water, then I'd suspect the heater matrix, and smell would be hot coolant.
