Andyoh

New boy porsche owner. bought a old 1998 2.5 boxster as summer fun project. I have a strange oil pressure problem !!!! have checked oil pump & drive and both scavenger pumps and found no issues there.
My problem is when i start from cold i have around 22psi oil pressure after around 5minutes at tick over pressure drops rapidly to around 4 psi and puts the oil warning light on , I have checked this with external gauge and thats whats happerning. changed the oil filter that had collased and will refill oil with 5w-40w at the weekend. cant beleave this this will sort my issues! Anyone out there got any ideas ?? Should i engine flush this first ?
Thanks in advance.