Low Oil Pressure Only On Start Up

[KRISKARRERA 2 1 Cars]

For a few months now I've had an oil pressure problem.

 

When the engine is left overnight, or for a good 10 hours, and you start it up the oil pressure warning light comes on for a couple of seconds.

 

First thought was that the sensor was playing up. Unfortunately there's no way it can be that because when the light is on those few seconds you can actually hear the painful metal against metal contact in the head!

 

When this started a few months back I had a s*ite oil filter on the engine so I guessed it might not have a non-return valve. But now I've fitter and Pug filter and I'm still getting the same sh1t.

 

Really got me stumped this one! Why would the oil drain out of the head?