What can make the oil pressure drop when engine's been running awhile? Can the turbo make the oil frothy and lighter? or is this a more ominus sign...
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Other than the usual drop in viscosity when the oil gets hot, it is possible that your engine is loose. When all of the parts get up to operating temp the bearings and other engine parts start to loosen up and let the oil pass by more freely. That is one of the signs of an old engine, the oil pressure is low showing that the engine is due ofr an overhaul. My engine usually shows about 60psi at start up. After a while and after the engine gets hot it will drop to about 45psi, 213,500 miles. In my jeep at start up I see about 12 psi after the engine gets to running temp the pressure drops to near 0psi, I hope it lasts through the summer, but it is old and used.

mine is the same way, 60 on startup and then 45 after warm. a newer motor, like my girlfriends 01 s10 will have much more consistent oil pressure. at startup and when warmed up, oil pressure is consistently at 60 or 80 pounds (i cant remember what the top marking is on the gauge)

This block had a complete remanufacture at 240,000 and it has 318,000 now. The oil pressure drop thing started 15,000 miles back after I replaced one piston and every gasket in the way. The bearing on #6 rod had plenty of life in it. oil pump was dropped at that time and replaced, although I would have liked a HO oil pump to go in.

Mines kinda like some of ya'll's. It's about 60 on cold startup, then goes to about 45 during operation once warmed-up. However, once warmed and idling (like at a stoplight) it will drop down to about 30. I just always thought it was normal Chevy thing since that's what my company truck did ( '00 Chev K2500 HD 4x4 ext cab w/Vortec 350) and my wife's Tahoe (4x4 w/ Vortec 350).

Just a little comidy, but I was concerned about the same thing "Low oil pressure on the guage" so I contacted the company that installed my new 6.5 td and the info that I got from them is that the new engines are designed to run on lower oil pressure and higher volume.
Electronic guages are not always reliable so you may want to install an extra mechanical guage or just replace the oil pressure sending unit on the existing guage. Not a bad idea anyway as your lift pump is run by a sender in the guage sender
You may also want to try an oil change with a differnt brand of oil and filter. Cut open the old filter and check for metal in the old one

Want to find where all the pressure is going? Drop the pan and pressurize the oil gallery with oil. You'll see where all the pressure is going. You'll be shocked at all the "internal bleeding" in a block. What kind of oil are you using? What are your oil Temps?

RotellaT for the last 20,000 mi. unknown oil temps

Get a lazer pointing temp gun and point it at the oil pan when she gets hot. Pick a spot not in the air flow.