Hi. First of all this may or may not be in the right place, and really has nothing to do with oil... But in a way it does.
I have a 99 Chevy Malibu with the 3100 V6. It has 221,000 miles on original engine and transmission. The engine has a knocking sound coming from the bottom (oil pan area) and it doesn't go away when it warms up. I've tried a thicker oil, and the noise got extremely worse. It is really bad, then towards the end of the oils life, the noise lightens. But when I change the oil, the noise is back. I'm running Pennzoil 5W-30 with a Wix oil filter. The noise was somewhat going away. That was when I floored it every single day. Well I started trying to drive it easier and ever since the car sounds like a diesel on a hot restart, but the sound fades a but after it runs a while. The oil pressure was 35-40 Psi on a cold engine. I haven't checked it hot. When I start it in the morning, the noise isn't there, but after it runs for 30 seconds the knocking noise comes. It sounds like marbles or something. I can get a video in the morning if you guys need to hear it. Sorry this is so long and thank you guys so so much for your help!