Fumes in cab, 1990 Toyota 4Runner

I am getting fumes in my 1990 4Runner SR5 6 cyl. I believe them to be exhaust or oil burning, not fuel. The condition does NOT exist when the windows are closed or when the fresh air vent is open or closed (recirculate).
The fumes exist in the cab only when I open a window (driver side), and is mitigated some when I open the fresh air vent. This may mean that the leak is not coming through the ventilation system.
What could be causing this?
My mechanic says it is worn valve cover gaskets, dripping oil onto exhaust system and finding its way into the cab, it does leak oil. This may be true, but how are the fumes getting into the cab in the first place? The fumes came on suddenly, after a rear window repair at the shop - unrelated?
The vehicle has been inspected by a muffler shop, no issues with the exhaust or manifold system.
The fumes are inscrutable, not strong enough to easily detect, but persistent exposure gives me a headache and dizzy feeling, and then I get polluted enough, that I cannot really smell it anymore, hard to trouble shoot.
Interior smells of exhaust, even when parked.
Thanks, Russ

I should mention that we inspected the rear window gasket and even added another gasket onto the tailgate gasket for an additional seal. I dont think the fumes are coming from the rear.

@russw “headache and dizzy feeling” usually means exhaust leak

Have your mechanic use an evap/smoke machine to inspect the exhaust system for leaks.

Cracked exhaust manifolds and leaking exhaust manifold flange gaskets are fairly common on some vehicles. Perhaps something just needs to be retorqued.

Thanks. Will look into that. The muffler shop that inspected it up on a rack, used a stethoscope with an open end to listen for leaks.

Anyway to mechanically distinguish what kind of fumes they are, exhaust, burning oil, etc. Thought of a residential carbon monoxide tester?