Oil Light

[Achilles]

I have a 97 Montero Sport XLS. I recently had an oil change done. After picking up the vehicle I noticed the oil light coming on. It comes on at low idle, typically at stop lights, when the RPM is down to just under 500. The engine also tends to run a little rough when the oil light comes on - I'm not sure if the oil light issue is caused by the rough idle or vice versa. As soon as I accelerate the light goes out. This happens more then the engine is warm. The car seems to burn a little more oil than I am used to but it's at 140K miles now so that might not be unusual.

The first time I noticed it, I stopped and checked the oil and it appeared to be overfilled. I let the situation go since I didn't think overfilling would be an issue but now I am concerned as the situation hasn't cleared up. I asked the mechanic to check the situation and he said he couldn't figure out what was causing the problem. I've continued to check the oil level and it's running within the marks on the dipstick. I've put a lot of miles on the car in this condition but now I am getting worried that I might be doing some damage to the engine.

I considered that it might be a defective oil pressure switch but I think other things are more likely since the idle is rough, too. I thought the car might need a tune up and had that done but the problem didn't go away. I suspect that the something is wrong with the oil pump - perhaps there is a shaft seal of some sort that is worn/blown? Or, since the light comes on only at low idle, there is not enough pressure being created by the pump to close the switch at that RPM. Could it be a clearance issue, as in a worn pump not pumping efficiently?

Any help is greatly appreciated!

Thanks,
Larry