2008 tahoe oil pressure is at 20 psi on start up and then drops to 5 psi at idle after engine warms up also will go to 20 psi at 2000 rpm and drops back to 5 psi at idle

5-10 psi at idle with a warm to hot engine is not unusual with a well used engine but is low enough to ring alarm bells if that is the reading at speed. The manufacturer will issue specifications for an oil pressure test but as a general rule the test would be made at around 2000 rpm and the oil pressure should be around the oil pressure relief valve lift-off pressure which commonly is about 40-45 psi but could be higher or slightly lower.

Oil pressure has little relationship with engine power as long as it still has some...

If the oil pressure is low the cause might be simple or it might be complex and expensive, but whichever it turns out to be, rectification should not be delayed even for a day...

this means a mechanical gauge test is required to check actual pressure warm idling if low below 20 psi then the oil pressure pump relief is stuck open or engine bearing are worn

55 is max (pressure relief)
20 is typical hot engine at 800 idle. and rises as RPM rises.
your post tells neither , temp, or RPM

I looked at info for 1500--5.7L engine.
6 psi @ 1000 rpms
18 psi@2000 rpms
24 psi @ 4000 rpms

Sometimes, you can use analog gage in place of oil pressure sending unit and double check oil pressure that way. If oil pressure is actually low, if high mileage engine, I doubt replacing oil pump will help. I just wanted to add, check oil pressure engine warmed up.

If the noise you are hearing is a deep hard sounding knock or rapid metallic rattle that vanishes as soon as the oil pressure increases then you could have either excessive bearing clearance at a rod and/or main bearing or an oil filter that is allowing the oil in it to drain back into the crankcase which is delaying oil from reaching the bearings for the first couple of seconds which is essentially like starting up the engine with no oil in it as it takes a bit for the oil to refill the filter and galleys before it can reach the bearings. It could be a hydraulic lifter but they generally make more of a ticking or clacking type of noise and take a bit longer to go away...and may even come back at random times. To find the source of these kinds of noises I usually hook up a mechanical oil pressure test gauge, allow the vehicle to sit overnight, then start it up and see how long it takes to build up oil pressure. I then allow the engine to warm up fully [say 30 minutes of idling] and then recheck the oil pressure. If the noise stops as soon as the oil pressure rises then I'd try a genuine Mopar oil filter first to see if that fixes the problem. If the oil pressure drops off to less than 10 psi at a fully warmed up idle and doesn't get much over about 20 psi at 2000 rpm then you may have excessive rod/main bearing wear which would require some more serious work to correct. If the oil pressure stays at about 15 psi at an idle and rises to over 35 psi at 2000 rpm then the bearings are probably in decent shape yet. Hope this helps a bit and good luck!

60psi cold / 30-45psi warm is ideal. I've seen (with that mileage) 45 cold/ 20 warm and that's acceptable - anything less start thinking about a new oil pump.

how did you clean the gasket off of the head? if you used a sanding disc the grit will eat up the bearings. are you reading pressure with a mechanical gauge? you have replaced all bearings??

6 psi @ 1000 RPM
18 psi @ 2000 RPM
24 psi @ 4000 RPM
this is chevrolet specs on oil pressure in your truck so you are within the range.

describe your idle problem more and i will try and help you with that

First we would need to know when this occurs. Asuming the oil level is at the full mark!. If the pressure goes up during acceleration and down at idle when engine is fully warmed up, then this is normal. An engine requires 10 psi of oil pressure per 1,000 rpm ( ex. 20 psi at 2,000 rpm, 25 psi at 2,500 rpm, and so on ) . If the gauge is moving back and forth erratically at idle , i would say the gauge is bad. If it drops to 0 while accelerating I would say your oil pump is bad. If from time to time it seems to work and other times it seems not to work, I would say the oil pressure sending unit for the gauge is bad. If the pump failed you would start to here a Knocking noise from the engine in a short period of time. Use the above scenario's to narrow down what it may be.

First we would need to know when this occurs. Asuming the oil level is at the full mark!. If the pressure goes up during acceleration and down at idle when engine is fully warmed up, then this is normal. An engine requires 10 psi of oil pressure per 1,000 rpm ( ex. 20 psi at 2,000 rpm, 25 psi at 2,500 rpm, and so on ) . If the gauge is moving back and forth erratically at idle , i would say the gauge is bad. If it drops to 0 while accelerating I would say your oil pump is bad. If from time to time it seems to work and other times it seems not to work, I would say the oil pressure sending unit for the gauge is bad. If the pump failed you would start to here a Knocking noise from the engine in a short period of time. Use the above scenario's to narrow down what it may be.

at a bare minimum of 15 psi