When I swapped gauge clusters I was advised that the oil gauge sending unit was one of the components best bought at the dealer.

 

I've got the same issue with mine. it sucks. shows 0-5 psi all the time. I checked it with a mechanical gauge a long time ago just because and it's got oil pressure. I don't worry about it. sometimes the gauge works, sometimes it doesn't. you'll know if it doesn't have any oil pressure.

 

Where did you connect the mechanical gauge at?