Try this site. . .

GM Parts Direct: Your direct source for Genuine GM Parts

The part number for a 2005 Oil Pressure Sender is 12584940

Put it in the search. If you don't mind waiting a bit, this site is usually 50% cheaper than dealer with dealer products/brands, real GM parts.

Had to change the sender in my 2000 just a few months ago. Suddenly fell to zero. It always did its preset when the key was turned on but after engine was started sometimes would register zero sometimes 120.
$50.00 at autozone.

At the risk of beating a dead dog...

So, it'* a consensus? Upon initial start-up gauge goes through self check (0-psi, max-psi, 0, max, 0) then proceeds to settle and remain at max pegged. To me this begs a diagnosis of faulty sending unit but was wondering if we're 99.9% in line yet? $38 at AutoZone (Duralast), cold as a witches teet outside so I'm looking to make this a one-time diagnosis.

Cheers,
Terry