oil pressure in 8.1

What should the oil pressure be in a 2004 8.1? Mine runs at around 50 pounds between 1500 and 2000 rpms and then drops to about 25 at idle. This is after the engine has warmed up. The motor has around 600 miles on it, and I have followed the break-in recommendations of keeping the speed under 55 the first 500 miles.
Also, when should I change the oil that came in the truck from the factory?
Does it make any sense to change the 1/2 quart filter for a full quart filter?
Thanks for any replies.

ive had 2 ,03 silverados 1tons with the 8.1 and both were the same , and had it checked everything was fine.nature of the beast