Low oil pressure on idle / good on road

Yestarday I changed oil in my mustang 200 ci. now I have Castrol Magnatec 15W40. When I started it today my pressure was on max, on road I have normal read but after 15 mil when I stoped my oil pressure was low. That's normal ? Befor I had Castrol GTX 10W40 and I didin't notice that...

It's fairly normal, as long as you have over 10 PSI at idle in that thing you'll never hurt it.