You should listen to the folks , they aren't far off.
Cluster gauges are a BIG issue on GM trucks, I replaced dozens when I was at the Chevy dealer 5years ago ( and those were warranty ) also saw a lot of issues with oil pressure sending units.
as for using a mechanical gauge, where were you tapping in ?
the proper way to read pressure is an adapter that replaces the oil filter, so you read pressure right off the main oil feed. (it's a temporary set up for diagnostics only )
and it usually takes a few seconds to reach any real oil pressure on a cold engine, and longer if you have just opened the oiling system ( filter change, sending unit change, etc.....)
one reason is as you read, the oil filters drain back. common problem, has been for years. if you are concerned with rattles and clatter at start up, welcome to the LSx engines, it's called piston slap, they have it, it's known far and wide, it's noise, thats about it.
For anti drain back , use AC Delco filters, they have the valve in them, there are several different styles, try to find the most recent update, ( the letter at the end of the part number, the higher the letter ( a,b,c,d......) the more recent the update.
if the pressure gauge was steady ( needle didn't bounce ) and it had good pressure, and there are no knocks from parts trying escape, I doubt you have an oil pump issue.
