oil pressure

I recently acquired a 93 Chevy G20 van with 129,000 miles, after changing the oil with 5w30 Castrol high mileage oil, I noticed that at about 50 mph the oil pressure stayed at about 50 lbs, but at idle the pressure dropped to about 30 lbs, my question, is this normal, or should the pressure remain constant???
many thanks,
Frank AKA crazyhorse357

Normal

Thanks a lot DF,
That's one more worry off my list.