Oil pressure light coming on at idle

Dan's gonna HATE this one.:D

In and old high miliage motor that may be gunked up you can run 50/50 of 50wt oil and transmission fluid to clean up some sludge that might be blocking the oil from rockers and even oil pump regulator valves.
Run it for a day or two, then change it (Filter change is a MUST)
I works about 50% of the time due to the heavy detergents in the trans fluid.
If you have high miles and low oil pressure and there is nothing you can do about it, then a can of STP oil treatment will shut your light off.

All the above is only temporary of course, but I have run cars for years after doing it.