What Makes My 1995 GMC Sierra lose oil pressure but not lose oil?

[Bri]

I've just picked the this truck up 6 months ago. When I first got it no problems, but I have noticed in the last month that the Oil Pressure gage drops significantly. When I first start the truck, it stays right at 45psi. After some driving around to warm it up, it will drop when I come to a stop. Depending on driving type it will drop down really low to around 15psi. For the most part I use it around town and it drops slowly, but after any highway driving when I get to a stop it drops from around 35psi to 15-20psi.
I thought for sure it was because I had a low oil level, but I checked and it was normal. I had it changed anyways and it continued to do it after that as well.
I've looked for anti-freeze in the oil and nothing.
I've heard that switching oil's to something like 10w40 might help, but I normally use synthetic so I didn't want to switch it.
I'm wondering if it's the oil pump because I have been noticing that I have been overheating at harder than normal driving.
I know replacing the oil pump is expensive, so I'm just looking for any other suggestions like could this be solved by replacing an Oil Pressure Sending Unit? Could it be worn engine bearings? Or is this normal because the truck has 132k Miles on it?

Any help would surely be appreciated!!!

Thanks,
Brian