Lick the glass.

Sweet?

Easy (fast, clear) answer. Good!

Coolant.

BUT hard and slow solution. Bad.

Probably heater core, but maybe you are lucky and it is another component?

Good luck!

You don't say what kind of smoke. Based on the smell % location, probably coolant => Bad heater core.

Your "ASE master certified)" mechanic should be able to tell antifreeze from oil by smelling the interior or touching the inside glass and tasting.

Test yourself by warming engine. Put a drop of oil from the dipstick on the exhaust manifold. Then a drop of coolant. Which is it in your car?

You didn't say the last time you check your oil so the 1/2-qt low could be coincidence.

According to Car Talk, breathing coolant fumes is hazardous to your brain cells. Your symptoms have been diagnosed by the brothers several times. It is not smoke rather it is a mist of water and antifreeze which is oily and sweet smelling.

Probably a leak in the coolant system inside the passenger compartment. Eg. heater core.

Keep in mind that you could pull the hoses at the firewall which enter/exit and connect them together, thus bypassing the interior. Until you get to fixing. I seldom have the heat on in my 240.

Am surprised that your mechanic would suggest a valve cover gasket considering the problem was inside the car. I think he would have been closer if he'd suggested door lock relay.
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1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.

It may very well be coolant, from a ruptured heater core. If it smells sweet - you mentioned syrupy - and seems to leave a taste in your mouth after breathing it, it's antifreeze. It might also be a simple hose connection to the core, so that's worth checking, but coming out of the vents usually means the core.
If so, it's not a fun job at all....