Pontiac guys.....oil pressure?

[av8or32]

I've got a 69 400 and when I first crank it the pressure is about 60psi (idle)then, once it's warm (about 200 degrees) it drops to around 20psi (idle), sometimes less. Is this normal or do I need to start thinking about a rebuild. When I rev it a little it kicks up to around 30 then back.