I have a 4.0 '92 Cherokee and it is currently burning oil. I only notice it when I'm idling and my brother can't see it when he drives behind me. I did a compression test and cylinder 5 was right at 120, while the other cylinders were all around 135-140. Also compression didn't change between a wet and dry test. I assume that the piston rings on 5 are bad. Do y'all have any thoughts?
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Leaking valve stem seals

 

could be^^.
Are the vacuum lines to the valve cover and air cleaner all buggered up/modified by chance? What's the air filter look like?

 

If the compression test pressures didn't change between the dry and wet tests then it's probably NOT the rings as that's the point of the wet test.

You didn't say how much oil you are losing. Any leaks?
Also. If you are only seeing it at idle and your follower doesn't see it are you sure it's oil smoke you are seeing rather than coolant ?

 

It has plenty of other oil leaks. I haven't owned it long enough to really get a good idea of how much it's loosing yet. To me, the smoke looks blue/grey. I'll probably ask some others their opinion on the smoker being oil vs coolant.

 

Is that better or worse then piston rings?

 

A seasoned mechanic should be able to replace them in a couple hours without removing the head.

For roughly the same amount of money, it's not much to remove the head yourself, other than being long and heavy, and have a shop rebuild the head. Cut and lap the valve seat, check and replace the valve guides as needed, replace the seals, check and cut the deck mating surface as needed.

With a couple tools, you can compress the valve springs yourself, remove the keepers and then the seals. Another tool and some compound and you can lap the valves yourself, assuming there's no issues that warrant cutting a new seat.

 

Air filter is fine, vacuum lines don't look too bad. Why do you think that would be part of my problem? It would be awesome if I only need to change a vacuum line lol

 

If the lines are there, and not modified, and your air filter isn't covered in oil, it's not likely the cause is vacuum lines.
A leakdown test can be performed to determine if the rings are the issue.