Oil pressure drop in low millage dt466

Just purchased my first (1998) dt466 21k miles. Oil pressure was at about 40 at high idle. Oil level was a little low so I added oil. I also added antifreeze as it was also low. Oil pressure then at 50 at high idle. After driving about 20 miles at about 45 mph the oil pressure dropped. Now high idle at 20. This is the first diesel I've owned. I can find any info (owners/service) information for first time diesel owners.