UK Car Repair: oil pressure low, lucas oil stabilizer, clear blue sky

Question
I have a 94 XJ6, and out of the clear blue sky, the oil pressure stated droping to zero when at idle.  Cruis speed, even at 20 mph, I get 40 lbs.  The oil is up and has recently been changed with filter to see if that would help the problem.  It didn't.  The only thing I have done different is that I was starting to pass a little smoke at start up only so I added some Lucas Oil Stabilizer.  Also is the sending unit connected to the gauge and the light or is there a different one for each.  What do you suggest.  Taking the oil pan off is a 8 hour job (lift the engine and all that).  Thanks, Buddy

Answer
I would say the problem is in the transmitter and is unlikely to be connected to the additive.  It is located under the inlet manifold closer to the front and is a little tricky to get to from above.  The modified(improved) unit, does both light and gauge, it is very simple and reasonably priced.