90 740 turbo

I was driving on the freeway today (AC on), and stopped off for some fast food. When I pulled up to the light, my oil pressure light came on - not steady, mostly barely glowing, but at times off or full brightness.
I turned off the A/C and the light turned off. I had just checked my oil, and it's as full as possible (top of the crosshatched are, but not over).
I continued driving, it came on again in the drive-thru, but only in drive/reverse, and not in park or neutral.

I changed from standard oil to synthetic oil about 10,000 miles ago, and changed it the first time at 3,000 miles. I've changed the filter 4-5 times already, using the Mobil 1 oil filters every time.... And just read a thread about mobil1 filters no longer having anti-drainback or bypass valves, and poor filter media.
I've changed the filters because I would notice that the oil light would stay on longer at startup over time.

I just recently tapped a new set of threads for the frontmost spark plug, and looked inside the cylinder.
Is it normal for it to be RUSTY inside the cylinder and on the top of the piston?
I've taken apart a few dirt bike engines recently, and they were VERY clean.
I also noticed that when I opened the oil filler cap, the cam didn't seem as wet with oil as usual - but the car hadn't been run for about a week.

I'm sensing a problem with oil delivery.....
with the turbo, my guesses in order of likeliness are:

-plugged oil filter
-plugged turbo oil cooling lines or something like that
-plugged oil pump pickup >.< Which sounds like BAD NEWS to me.

__EDIT__
I forgot to mention this, and I think it's important. Lately, my oil dipstick tube has been popping loose. It had popped out on me a few times, and I added an extra o-ring to it. It's now just popping loose, not out. This seems to me like there is high oil pressure where there shouldn't be.


Thanks for any help,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond