Sounds like a cam cover gasket is leaking to me, I've got the same problem ,
just need to find time to bring it to the shop.
From what I've heard it reasonable common & reasonably cheap to fit.  Just
use genuine Subaru gaskets.

I have a 2001 Forester with about 140k mi.

I have been smelling oil when I am at red lights for about 6 months.  I
decided to find and fix it.  It lead me to leaking boots around the spark
plug holes.  The boots inside the valve cover, not the spark plug boots.

I replaced the 2 on the passenger side that were obviously leaking.  But I
still smell some oil after many miles.  I then replaced the valve cover
seal on that side, and got some new rubber washer things.  I don't see any
oil.  But I still smell it a bit.

I need to do the drivers side now.  But it was not leaking nearly as much
as the passenger side.  Neither leak put a drop on the ground.  It always
was just the bad smell and slight smoke.

Anyone have experience with this part?  Anything else I should replace
while I am at it that might go bad at the same time?  Any trick to doing
the driver's side?

Thanks,

CL