serious trouble??

Many cars do this. I've had a couple car that even if you pulled the dip stick it would effect the running of the car. Some are real sensitive to oil pressure drop.

Thanks Matt. I don't see how oil pressure would affect idle speed or smoothness since the function of oil pressure is to pump oil to engine parts and less pressure would only mean less efficient oil delivery to those parts. How could a drop in pressure affect anything like compression , air/fuel mixture, spark timing, etc? Not argueing with you but what am I missing? Do you really think it is normal or should we dump this car before she blows?

You could do a compression test on each cylinder, also check for water in the oil or oil in the coolant for a possible head gasket leak. The older model engine had HEX head bolts that get brittle around 150k miles (maybe more) and can cause a leak. You could replace the head bolts to Star or Tork or Torque type head bolts before they get weaker and you blow the head gasket.

This is nothing to do with oil pressure. This is everything to do with the emission system. A loss of vaccum from opening the oil filler cap has direct and detrimental effect on the idle control valve and thus causes poor idling or even stalling of the engine in some cases. Along the same line, more subtle idling problem is most often a result of leaky cam cover gasket, a very inexpensive and effective fix, not that you have this problem anyway. In your case, there is nothing to worry about.

ICV is electronically controlled from the ECU/OBDI on these cars, not vacuum from the manifold.