Chevrolet Repair: Oil Leak, napa autocare center, intake manifold gasket

Question
I'm the original owner of a 1993 Chevy K1500 pickup with a 5.7L engine. The truck is just coming up on 37,000 actual miles. Until 2 years ago I worked out of state and the truck just sat in the garage for 2 weeks out of every 4. At around 15,000 miles the engine developed an oil leak. Drops of oil are on the back of the oil pan, the torque converter cover, and bottom of the starter. One Chevy dealer and one service station owner think the leak is from the rear of the intake manifold. I took it to a NAPA Autocare center with a good reputation (with AAA members) to have the intake manifold gasket replaced. Their diagnosis was that the intake was just damp, not leaking bad enough to be the source of the drops. They looked at the oil sending unit and distributor and ruled them out too. They think the rear main seal is the source and said there was no harm in leaving it as is. My own attempts to check into it further, mostly on websites, have me questioning if the source might be the oil filter adapter gasket, apparently a common problem and a much less expensive repair. The Chevy dealer and NAPA didn't mention this as a possibility. Any suggestions on what I should do to resolve this? Thanks for giving your time to strangers!  

Answer
The oil filter base, and even the cooler lines if you have them can leak, but they are on the opposite side from the starter usually.
There is the possibility of a valve cover gasket, or the oil pan gasket.
For the rear main seal to leak onto the starter is hard, but i guess not impossible.

Get in a dark place, or check at night with a good flashlight, rather than a droplight. You can concentrate the beam better.

Van