Why all of a sudden is my oil pressure dropping?

[JoetheMobster]

Hey folks,

Just changed the oil in my truck for the first time (haven't had it THAT long, so I hope it was changed at least once, before I got it...hahaha) Anyway, after changing the oil and filter, I fired 'er up, and looked for leaks around oil filter, and drain plug. Seems to be none. So I decide to take the ol' pig out for a boot, and I notice that my oil pressure gauge is reading just above the "half" mark as I'm pulling out of the driveway. As I'm heading to drop off the old oil, I notice the pressure gauge is dropping slowly! When I get back to the house, the gauge is just below the "quarter" mark!! What the heck is going on here? The dipstick says that it's topped up, and there seems to be no leaks. Anyone have any suggestions? I'd hate to seize the motor, as I don't have the space, or the money to do a rebuild, or replacement.

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1970 GMC 1500, 350ci, 3 in a tree

http://members.tripod.com/~Jungleman_2/1970GMC.html

Vancouver, B.C.