What does this mean? Is it a bad thing?

I used to get right around 70-72 PSI at 3000 RPMS before the rebuild, and now its right around 65 PSI. Idle is still the same as it was before at around 20 PSI.
I was using Mobile 1 before and now Im using regular oil, not sure if that has an effect on things.

Should I be concerned?

 

new oil pump?

change the oil and see what happens. but as long as its over 50 you should
be ok.

 

Yes I got a new oil pump as well..maybe that is why its lower now.

Ill be changing this oil at 1000 mile mark and we'll see then.
Is synthetic thinner/thicker then regular oil? Would it cause a change in pressure?

 

quite possibly.
I put an oil pressure gauge on a b18a recently, average pressure was 90!