2000 xj with 71,000.
I got good oil pressure on a cold start once at hot idle it will drop to about 20. Driving hot on the highway it will be at about 50. I have low coolant that is gray colored. Although my oil is at the right level and looks perfect. I was afraid I have a cracked head but now I'm not sure. Somebody figure this out
Thanks

Start by doing a compression test.

Did some other tests no bubbling with radiator cap off and no white steam coming out of exhaust and no crusty / milky substance on the oil fill cap.

You oil pressure is OK. Your mileage is a little low for the cracked head, but some do manifest as low as yours.

You could change the coolant and use a Prestone Super Flush kit while changing it.

Put Prestone or Super Tech back in, 50/50 mixture. After the level stabilizes, monitor it for usage.

Will do
Thanks for the input

my gauge was reading 18-20 psi at hot idle today I hooked it up to an oil pressure gauge which read a consistent 28 psi. 28 at hot idle normal/good? I'm going to replace the switch under the hood and see if that solves my problem. my question: is 28 psi good for a hot idle?
thanks

For a stock 4.0l: 12psi Or higher at idle and about 40psi at 2200 RPM.

How long and how many miles on the last oil change?

Like grainofsalt said when was your last oil change and what brand of oil filter do you have? The best filter for our 4.0's would be Mopar, Wix, or Napa Gold. If you have a crappy filter such as Fram or any other cheap off brand can cause low oil pressure too. But like you mentioned it was reading off when you put on a mechanical gauge. So changing your sending unit is a good place to start. Also if you can get one from the dealer they seem to read better then an after market unit.

Indeed: 12 psi or higher is within the OEM spec normal oil pressure at hot idle, using the OEM spec engine oil (10W-30). You are clearly above that. My own 4.0 reads 19 PSI per dash gauge at hot idle.

Of course, oil pressure is not a test of your head...
As I understand it, coolant loss into the oil is the most common first symptom of the failure mode "crack" of our 2000 4.0 head. So, your clean oil cap, and clean dipstick are good signs.

Are you sure that you are actively loosing coolant? If so, look for drips off the front of the engine. This could be a sign of your water pump going bad.

2500 miles since last oil change and it looks clean. Well I think I'm clear of a cracked head but sending unit is going to be swapped out and ill see what happens and I do believe I have a mopar oil filter on there. I can't honestly say if I am actively loosing coolant I know I'm low but as someone said earlier I'm going to drain coolant and see if I loose anymore. Thank you all for the input

What about pressure checking cooling system and do cap as well.

That's new. (grey coolant) I never heard of a trans cooler leaking, but I might check my trans fluid just on GP.

Seems CherokeeJake1 (the OP) is not sure if he's actively loosing coolant. If no coolant is being lost, then a pressure check is not needed.

Step 1: Is coolant being lost?
Step 2: If yes, then: from where?

CherokeeJake1: I would not drain and refill my coolant until knowing the answer to Step #1. Just refill it, and watch closely for future loss. Then, if there IS active loss (more than 1/2 gallon), proceed to Step 2. If the leak is not obvious, then a radiator pressure check is the way to go. Any Step 2 repair will require coolant drain anyways. Also, I'm not sure you need to swap out the oil pressure sending unit.

Not necessarily. I had good oil pressure, clean oil cap, dipstick and no signs of coolant in oil, but my head was cracked.

To the OP. I finally found the leak when I opened the oil cap immediately after a 15 mile highway trip with the engine off. There were two tiny pinholes of coolant bubbling up into the head between the 3 and 4 cylinder. I could only see it with an LED light.

So, you found the crack by witnessing coolant leaking through your cracked head into your oil.

To OP:
Step 1: Is coolant being lost?
Step 2: If yes, then: from where?