First off, the YJ gauges are noted to be inaccurate. So this is most likely your problem.
If you want to investigate it further, buy a cheap manual oil pressure gauge like an autometer, and install it in the pressure sending unit hole. Which is located down between the distributor and the starter. A manual gauge has the 1/8th inch plastic line to give you a more true reading of the actual oil pressure.
If it is low, it could be that the oil pickup tube is clogged or partially clogged (it has a screen on the bottom of it). Its accessible by dropping the oil pan. Or, the oil pump is going out on it.
But I would think that its a gauge issue if you have been having this problem for 3 years and havent scattered that motor yet. 40 psi is normal, down near zero is extremely dangerous. Unless your ultimate goal is to see how tight you can get that motor to sieze up.