Burning Oil smell after head gasket replacement on Subaru Forester 2001

Maintenance/Repairs
1

Had the head gasket replaced and metal machined due to HG failure. 2001 Forester, 120K miles. After I got it back, a bad oil smell. Cleaned and sprayed it, still there, though less; seems to be on drivers side. Comes in to cabin from the air vents below windshield when stopped.

Could it be the line from manifold vacuum to brake master cylinder ie servo amplifier? Or something else?
Should I worry? The car seems to get about 15 mpg, used to get 18 mpg, and the checkengine MIL is on.

Thanks if this reminds you of anything

2

You use the term “head gasket” in the singular. Does this mean only one was replaced? The car has 2 head gaskets and when a head gasket is necessary both should always be replaced.

As to MPG, a number of things could be behind that. The first step might be having it checked for any codes at the least. The chain type auto parts houses will do this for you free of charge in almost all states.