JT, my '91 has been doing the same thing for several years now. Might have something to do with the 200k miles it has now. Anyway, there is a vacumn port just aft of the throttle body that has a tendency to plug up. Clean it out and that will help a little. Won't cure the problem as it is probable due to age & miles. Just tired. Mine still passes emissions and runs good. Not as strong as it was 10 years ago, but neither am I. It still gets me where I want to go. Just replace the filter more often. I buy cheap filters but may break down and go the K&N route.
Short version, don't worry about it.
'91 YJ, 4.0L 5sp, Atlas II, RE SOA + 2"BDS, 35" Explorer Xterrains, ARB's & 4.56 gears, WARN 9000i
This is a reply from another similar post, this may help...--------------------------------------------------------------------------------
Regarding oil in the air cleaner.
I have been reading this same topic from forum to forum. Everything from removing valve covers to rebuilding motors because of warn or broken parts. Here is a suggestion from a Jeeper that had and fixed that same problem. I now have well over 225,000 miles on my daily driven, rock crawling, 1989 YJ. (With no oil in the air cleaner) On the 258 “Big92Jeep” is correct to say that it uses a pcv valve in the valve cover, but before any serious part removal, try this. With help from the one non-salesperson at Jeep, I fixed the problem with a $1.99 can of gumout. With the engine running you can lift the pcv valve out of the valve cover and put your finger over the end and you should have some serious suction. (While revving the engine) If not, the pcv valve does sometime clog, (When you shake it, it should rattle) To clean he suggested spraying with gumout or replacing (if necessary). But the culprit is usually inside the hose that runs from the pcv valve to the intake manifold. If you removed the air cleaner you would find a split in that (approx ½ inch outside diameter) hose. It connects two hoses together. Inside is a small orifice (approx 1/8 inch inside diameter) that gets clogged very easily. (Trying to remove the two pieces usually ruins the orifice)(It gets brittle)
Before you remove anything, with the engine running, spray gumout into the pcv valve or remove it and spray directly into the hose (keeping the rpms up, so the engine won’t stall) Depending on the severity of the clog a little or a lot of gumout will be used. (Sometimes a piece of piano wire is needed to unblock the orifice) You should notice immediately a difference in the amount of suction at the pcv valve. Reinstall the valve, shut of the motor, open and clean the excess oil from inside the air cleaner. Replace, if necessary, the air filter (this is a good time to install a K&N filter) Remove the little filter, spray it with gumout, replace it and put back all the other parts. This completely fixed my problem. Two years to find and $1.99 and five minutes to fix.
Note: With this line clogged, the pressure inside the valve cover forces oil up through the rear hose into the air cleaner. And with so much oil inside and leaking out of the air cleaner, it leaves room for shady mechanics to suggest some serious $$$ repairs.
That repair was done about 50,000 miles ago, and just to be safe; I spray out that hose along with the carb during periodical maintenance. The problem never came back.
Give this a try. It solved my problem, and at $1.99 for a can of gumout, this might be the cheapest suggestion you’ll get.