I just replaced the oil pressure switch on my 4.0 Cherokee this evening and I am still running low (near zero) pressure at idle once the engine warms up. Could running a 5W30 versus a 10W30 oil make the oil pressure drop. This problem occurred after getting my oil changed at a dealership a week ago.

I am not loosing any oil and there is no tapping coming from the engine. I'm a little perplexed. It's a 2001 with 72k miles on it.

 

I also would be concerned. I guess the next quick and easy step would be to change the oil and the filter. Use a heavier oil and see what changes you notice. If you still have low pressure it is time to drop the pan. Change the oil pump and plastigage each of the main bearings. Worn bearings will lower oil pressure. See if the plastigage tells you anything. If the bearings are worn then you can change the bearings without a complete rebuild. I have done that a couple of times over the years and got good results as far as continued use out of those engines.
Take it one step at a time. First the oil change.

 

hope the engine is ok!

 

First... no changing from a 10w30 to a 5w30 isn't going to drop oil pressure once the Jeep is warmed up.

The first number in a multi-viscosity oil is the viscosity index at cold temps. The number after the W is the viscosity index at operating temperatures. So in equal quality oils a 10w30 is the same viscosity as the 5w30 at operating temperatures.

The oil pressure sender is in a bad place for damage. They are kinda prone to getting hit when people change the oil filter. I would examine it carefully for damage, also inspect the wire going to and make sure it is plugged in all the way. Secondly, if your dealership used a low quality oil with a shoddy additive package it may be dropping below the correct vicosity at operating temperatures. Our Jeeps operate regularly at temps higher than alot of vehicles on the market. The dealership may use an oil not robust enough for the rougher service our 4.0l delivers to them.

Also I would be VERY concerned about any dealership bulk oil service to begin with. Unless they can give you a guarantee that the oil meets API-SL standards stay away. With the new API-SM standard you simply aren't getting the correct protection for your motor from that oil.

 

I would suggest putting a real guage on the engine and see what he numbers really are. Those needle guages can be flakey and not tell the real story. See what it has at ilde then rev it up a bit and see what you have.

 

That's the next thing I'd check as well. You could have a bad dash gauge, and that would be an easy fix. HTH!!