How many miles are on the vehicle? I've never seen a distributor bushing issue on any vehicle, and I'm old.

 

197xxx

I have a naive question: are mailfold intake gasket and plenum gasket the same? If no - should I replace both?
And what is the best brand for gaskets to purchase it terms of price/quality?

 

When you look down the TB holes in the manifold, you are seeing a plate bolted to the bottom of the manifold casting. Once that plate is on, the manifold is a complete assembly and gets bolted to the motor. Here is an intake gasket set. The top one is for that plate. The plate is sheet metal and can flex and leak over time, allowing crankcase oil into the intake manifold and be visible down the TB holes.

 

As mechanic suggested I replaced the distributor cap - from my point of view it does not look bad (see the picture). All symptoms still exist - so ignition system is not a problem.
I also did not notice any problems with distributor shaft, just some backlash.
But I got a second chance to look into the mainfold through the throttle: the plate below is completely covered by oil. It is not a pool, so far, but the area I could see has an uniform "golden" color.

 

New and disappointing updates. The misfire seems to be gone after the intake gasket replacement.... But it was gone for ~600 miles and came back recently and it gets much worse.
It occurs on all RPM up to 2000. I believe, on idle the misfire correlates with light "clicks" near/in a distributor cap but all pads in the cap and wires look OK.
And, finally, I got check engine light which gives P0308 error code (using the OBD scanner at Autozone). But If turn an ignition key twice - I came up with P1281 (Engine is Cold Too Long) which is very confusing.

Any suggestions? What I should dig next?

 

Swap injector on 6 and 8. See if the misfire moves.

What temp is your thermostat? When was the last time it was changed?