Oil Question

I have a 408 that has a roller cam with solid lifters and roller rockers. It may have about 1000 miles at most on it since it was built and during that time it’s always had 20-50 penn grade oil. The last oil change I had to buy partial synthetic oil. I’m noticing that oil pressure is lower with this oil. Probably 15 lbs cold and 10-15 lbs once warmed up to normal operating temps.
Is this a normal thing and what happens if I go to full synthetic 20-50?
Thanks Rod