If your vehicle is showing burning oil smell coming through the ac vents, here are the most likely causes and the parts you'll need to fix each one.
The valve cover gasket degrades over time, allowing engine oil to seep out and drip directly onto the hot exhaust manifold. The resulting smoke is drawn into the cabin through the HVAC fresh air intake near the windshield.
Oil spilled around the filler neck during a recent oil change or top-off can pool in the engine valley or run down onto the exhaust. This will burn off and smell for a few days until the residue is gone.
A blocked Positive Crankcase Ventilation (PCV) valve causes excess pressure to build up inside the engine. This pressure forces oil out of various seals and gaskets, which then drips onto hot engine components.
Seals around the variable valve timing solenoids or camshafts can harden and leak oil down the side or back of the engine block, eventually hitting the hot exhaust pipe.
On turbocharged vehicles, the oil feed or return lines can develop leaks. Because the turbocharger gets extremely hot, even a small drop of oil will instantly vaporize and create a strong burning smell.