The 3000 300 cpu has a funny double scsi looking
connector on the cpu card.  But, the actual
connector pins are tiny edge connector fingers.
It is almost impossible to get anything in there
to clean them.   Seating and unseating several
times seems to wipe them clean enough to work.
I doubt there is enough clearance to get even
a thin q-tip into it to wipe a bit of vaseline
across the contacts.  But, I can appreciate the
vaseline approach... I have been using that on
WWII era radio equipment, for years.  And no,
no pencil eraser cleaning of computer bits....
(:+}}...

My guess for a possible cleaning tool would be
a thin putty knife about 2 inches wide, with
a piece of soft cotton T-shirt taped to it and
then dipped into some isopropyl and inserted
into the connector....

Bob