Hello, I have a 2000 Pontiac Bonneville with 3800 engine. Recently, I started to noticed that the oil pressure will drop while I am driving. If I let it rest, and start it up, the oil pressure would go pass the 40 psi mark but stop before the 60 psi mark. Then when I drive it around for maybe 15 minutes, the oil pressure starts to decrease slowly until after another 15 minute, it would decrease pass the 40 psi mark and would continue to decrease until I turn it off once it went pass the 20 psi mark after another 20 or 25 minutes.

I changed the oil, oil filter, and the oil sender unit

I also think the pressure starts to decrease after the temperature gets to 203 (The pictures shows more degrees but its at 203 since the camera angle)

When I changed the oil, there was residues inside, (Small black particles maybe?) but the oil that came out of the oil filter is darker than the oil that came out from the oil pan.

Also a couple years back, I changed the front head values cover (the piece with the valves that goes on the on top of the cylinders) and it didn't work only until recently, the gas pump was changed and it started to run again on the dime.

The picture I had attached is when I drove for nearly one hour and the pressure was at 20 psi mark

Hopefully, these help you decide whats wrong.

Nothing is wrong. Cold engine=high oil pressure. Hot engine=low oil pressure(20psi or so) This is normal.

I agree with Mike, this is normal behavior, especially if oil pressure jumps right back up with a blip of the throttle. The general rule is that anything at or beyond 10psi per thousand RPM is fine. Just make sure you keep up with regular maintenance, and are using the recommended oil weight.

And also remember, even though you replaced the sender, gauge accuracy has never been one of GM'* strong suits.

Thank you for the reassurances, I do keep up with the maintenance, I just freaked out little when the needle actually arrived to red area recently so I thought something was terrible wrong and overlooked. However, I tested drive again and found out it didnt touch the red area again but hovered around the 20 psi mark. And thank you, Ill rely on my feeling and what little hearing I have left to notice it beforehand.

I was also reading some forum on here and some of the people recommand using 5w - 30 oil since its a old car? I have in 10w - 30 oil in my car. Any positive and negative aspects of it?

Thank you for reading

GM recommends overall 10w-30 for your car. For cold winter conditions you could use 5w-30, but personally I use synthetic 10w-30 year round with no issues.
If you have your owners manual, you can refer to 6-13 & 6-14 for further oil specifications / recommendations.

If he switches to a lighter weight oil, he may see slightly lower oil pressures.

Only at lower temperatures if it'* 5w-30. Operating temperature should remain the same.