Here are the results of the compression test:
1. 138 psi
2. 137 psi
3. 132 psi
4. 137 psi
5. 130 psi
6. 128 psi
The plugs look normal on all cylinders with exception to 5 & 6 had (wet) oil residue around the threads at the base of the plug and the seats on the block; however, the plugs were not oil fouled. So, I may have some blow-by.
A couple weeks ago the engine started making a subtle 'whine' similar to a noisy automatic transmission and I noticed my oil pressure had dropped considerably (in half). With a manual gauge screwed in, the engine produces 0-18 at idle/stoplight, and ~30 to 45 under power (and at higher revs). Prior to all this, oil pressure was consistently in the 70-80 range on my interior gauge; however, at a stoplight or idle would fall to ~30-45 psi.
The mechanic that usually works on my car suspects the oil pump is bad. What other things should I look for to pinpoint my problem or confirm this? Otherwise, the car still runs good/strong and has smooth idle but I'm afraid to drive it with the low oil pressure and the subtle whining. There is no visible smoke or burnt oil smell coming from the car.