2012 oil burn smell

Hello everyone,
I have a 2012 eCo and I get this oil-burning smell from the engine bay but no obvious sign of an oil leak. The car currently has 149k miles and at ~145k the intake manifold and valve cover were replaced. The intake manifold replacement is a Dorman but I'm not sure of the valve cover brand since it was a repair shop that installed it.
I replaced the intake manifold no issues with that effort. Unfortunately, when I was installing the Dorman valve cover, I stripped one of the threads of the head hence taking it to the shop for repair; also why I don't know the brand of the valve cover.
Since that repair from the shop, there is no sign of an oil leak around the valve cover nor are there any drips from the bottom of the engine. The oil-burn smell is somewhat mild when the car is driven normally. But on the occasions when I'm in a hurry to get to work, the smell is a bit stronger.
When I use the heater, the smell comes through the vents but when I use AC, I don't get the oil-burn smell. I'm thinking the blower may be an issue but why only when using the heater?
Possibly there is a coolant leak at the heater core but that doesn't seem to address the oil-burn smell from the engine bay and yes, I had to add some oil this morning. And I don't get any smoke from the tailpipe which is puzzling why the decrease in oil.
There's plenty here, apologies, but if anyone has suggestions, please advise.

Thank you.