Try this I found on another site...this is another blokes story so I take no credit for it
V6 3800 and V8 5000 Oil Pressure Problems (my VS Caprice, 160,000km)
The Oil warning light came on and the car stopped - that was the good news. The bad news was- there was NO oil pressure. When I started it again the top end was rattling badly - lifted deflated, etc. Recognised mechanics all said change the oil pump after checking the filter. Nothing wrong with the filter so off with the oil pump, Hmmm brand new! no wear! still no oil pressure. checked the "Gregorys" manual, followed checks. Finally had to drop the oil pan for inspection, read on for procedure.
Caution - ALWAYS THINK SAFETY FIRST and DO NOT climb under vehicle unless well supported by approved methods. Approved Methods do not include the use of house bricks!!
Step 1.Remove the front suspension crossmember(x-mmbr). Jack or Raise the front of the car and support on the chassis under front door pillars with car jacks, not on car ramps) Then lower the x-mmbr(easiest when 4 x-mmbr bolts undone and lift the bolts out of the chassis rail by bending back the tab on top of the bolt, slowly lower the xmmbr a few inches and support). "Lifting the engine may not be required but this is how I did it, and suggest undoing the manifolds is still a good idea"
Step 2. Drain the oil (1 inch socket), if the oil is new you may want to recover it into oil drums not your grotty sump tray.
Step 3.Undo the two lower engine mount bolts and exhaust manifold bolts and checking the wiring and hoses all the time (esp. aircon) lift the engine with a quality sling under the harmonic balancer(may not be a good lifting point but it works!)and the oil pan "can" be removed without removing starter, aircon, etc! saves time. use a 3/8 & 14mm socket to remove the oil pan bolts.
Step 4. with the oil pan out you should be able to see the cause of your problem. Crud blocking the sump pump pickup and filling the "dead space" in the bottom of the sump. NB: This problem has come about by Holden trying to resolve oil surge problems. There is a metal "cup" over the steel gauze (read: "crud filter". Crud is caused by using "Stop Leak" products usually by car salesman fixing gasket leakage or compression problems) which slowly fills with crud and does not fall out when the oil is changed.
Step 5. Remove and wash clean the pickup (1 x Main bearing bolt and 2 x 3/8 socket sized bolts) Cut 3/4 of this "cup" away, clean the rest of the crud out and smooth the edges of the cut joint (I marked a line and used an angle grinder to cut the cup off) then refit it in the engine and torque the Main Bearing bolt back up using a torque wrench with correct setting.
Step 6a. Clean the remaining crud and your "recently serviced" oil from the pan. NB: Important point for your NEXT SERVICE - note how the Sump Drain Bolt sits on the side of the pan and does not drain all the oil and crud from the sump when doing and oil change. I suggest you raise the left side of the car when draining oil - (maybe drive it up a car ramp at front and jack the back? or change gravity 30' for a few minutes).
Step 6b. Refit the pan using a new neoprene gasket from Coventries (about $17:00, * aids their xmas pissup fund) using red high temp RTV to hold the gasket to the bottom of the engine while lifting the pan into place.
Step 7.Checking all cables/hoses clear raise(hydraulic jack) and rebolt the x-mmbr in place, these are locking nuts and should be thrown away and replaced with new bolts and nuts(see * in step 6b).
Step 8.Check the rest of wiring/hoses etc and Lower Engine and rebolt.
Steps 9 to 15. Remount manifolds, lower vehicle, (I installed a 600 kPa gauge beside oil pressure switch (and 1000kPa gauge in fuel delivery line) for future reference) Fill 'er up with oil/water etc. and check engine pressure with a gauge when starting.