no oil pressure til engines hot

I've got a 1995 F150 that has 138,000 miles and a month or so back I noticed that the engine would run fine until it completely warmed up, then it would sputter when sitting at the light, and the longer I drove it at one time, the more sluggish it got. It runs fine when it's cold..
However, I knew the oil was long overdue for changing, so when the oil pressure started taking longer to build up, I changed the oil and filter,,now,,here in Michigan, U.S., with cold weather (still!) in the morning, it can take 10 minutes before the oil pressure needle moves, and then if you drive it, as soon as you step on the gas, at all, the needle drops to zero....once the oil is hot, it comes right on and stays on as long as you don't drive it too long, because then it goes off again, and stays off til the engine cools off again.
this is driving me crazy...not to mention, ruining the engine,,,,any help would be very much appreciated!
Ryck

It might be your oil pump,it really could be a number of things. Oil sending unit is dirty, your oil is so thick it won't pump,you might have some kind of sludge running in your oil system, or even a gasket that isn't sealed like your oil pan gasket. Check some of these things out. Good luck

sounds like the oil pump is bypassing , when oil is cold it is thick and the pump turns , causing high oil pressure . the oil pump has to dump the oil (or should i say bypass) so you dont build up too much pressure and blow up the oil filter . the bypass valve and spring is in the pump, so lets say the spring is weak ....this will cause the pump to bypass early . as the oil warms it thins and has less pressure, the pump relife valve and spring overcomes the pressure (closes) and alows pressure to build up. the only thing that does not make sence is no gauge indication in the mourning....... ps i have the same problem ,i will be changing the pump b4 i drive the pickup 95 f150 300cid

What brand of oil filter are you using?

appreciate all the help with this and apologize for the delay in responding...been hectic as of late...
as far as the oil being 'too thick, I just changed the oil the other day as mentioned in the post,,but I have wondered if, as you said, perhaps there's sludge still in the motor and it may have partially plugged the oil pump sending unit?? Since it was so long since I changed the oil, would it be helpful to change the oil again to help remove the sludge? as far as the gasket, it seems if there was a bad oil pan gasket I'd have a leak, at least some 'drips' somewhere, which I don't...
What puzzles me is that if the oil pump is 'by-passing the oil' due to high oil pressure, then why does the sending unit show 'no oil pressure'?,,,which you mentioned as well,,,I wish I had the option to 'not drive til I fixed it', but it's the only vehicle I have and I have to work,so,,,being as careful as I can, I have to drive it..
the oil filter is a 'Motorcraft'...
What's sorta weird, but nice, is that yesterday I had no choice but to use the truck to pick up some material for a job I'm working on, and planning on having to pull over many times, I was surprised to find that driving it 50-55 on the x-pressway it never, in 12 miles dropped pressure,,,,the truck sat for a half hour or so and then drove it back and never did I see the pressure drop at all...before, the day before actually, I only drove it down the road two miles and the oil pressure started dropping when I stepped on the gas....
Could it be the oil pump sending unit was temporarily plugged?
thanks again for the suggestions,
Ryck

You may have sludge and debris floating around in the pan thats partially clogging the pickup screen at times.(sludge, old pieces of gasket, peices of valve stem seals...ect). Or it could be a mechanical problem. the best way to tell, is to install a real oil pressure gauge, then youll know exactly what the oil pressures doing. The "sending unit" thats on it, is nothing more than a switch. Anything above about 4# of pressure , gives a half scale reading, much less, and the gauge reads 0.Good luck.
Later
Ed