New member old questions

Hello all. I'm new to this forum annd have a few questions. I've tried searching the answers and had no luck.
I have a 2000 Gmc sierra 4X4.with a 5.3 L engine.

While driving on the highway, my oil pressure guage dropped to zero. I pulled over and shut the engine off. I checked for obvious visible problems and could find nothing. I started the engine and everything worked as normal. Oil pressure was back up. I have both guages and idiot lights. There was no unusual noise from the engine.
I continued drivine for another 40 minutes when it happened again. This time I pulled over and shut the engine off. Again a visual showed nothing. I restarted and the oil pressure came to normal but the engine lifters were noisy. I shut the engine again.
I trailered the truck home and it now runs with normal oil pressure and slight lifter noise. I have had it idle for an extended time with no change.
The truck has 415 thousand KMs. on it.

Any ideas??

I have acces to a 5.3L engine from the same model truck, but a year older. Will they swap??

Thanks

Welcome to AF, get a known accurate oil pressure gage tapped into the engine and see what you have.