Conventional or Synthetic Oil?
Newbie
Joined: Sep 2018
Posts: 6
Likes: 0
From: Texas
Year: 1997
Model: Cherokee (XJ)
Engine: 4.0
It really depends on the condition of the motor...
I have customers that have had no problems and ones that have had to replace seals, gaskets and etc.
If you have alot of miles and suspect the condition of your seals and gaskets, then stay conventional.
My main concern would to make sure you are staying with an oil with the correct amount of Zinc and Phosphorus. You want to make sure the oil meets the older API-SL rating.
I have customers that have had no problems and ones that have had to replace seals, gaskets and etc.
If you have alot of miles and suspect the condition of your seals and gaskets, then stay conventional.
My main concern would to make sure you are staying with an oil with the correct amount of Zinc and Phosphorus. You want to make sure the oil meets the older API-SL rating.
Senior Member
Joined: Jul 2013
Posts: 523
Likes: 99
From: PA
Year: 2000
Model: Cherokee
Engine: 4.0
Old worn out seals cause leaky seals, not synthetic oil, not conventional oil.
If you want to fix your leak, then you need to replace the gasket(s) that's leaking. As for oil, I use synthetic in everything I own (old and new). There is no shortage of oil related posts on this forum, and every other forum arguing one weight oil vs another, synthetic vs conventional, sugar vs saccharin, crunchy vs creamy...etc, etc. Run whatever oil is going to make you happy, but personally I wouldn't lose sleep over 1 qt in an oil change.
Old fart with a wrench
Joined: Nov 2010
Posts: 14,398
Likes: 740
From: Manlius, east of Syracuse, NY
Year: 2000 XJ Sport & WJ Laredo
Model: Grand Cherokee (WJ)
Engine: 4.0L
Hell, for many moons if your engine wasn't using a quart between oil changes, you had to worry. I'm talking 50's here. They had things called "road draft tubes" where crankcase blowby was vented underneath the car. It took PCV valves in the 60's to change that. Now with the same usage, you have to look for a leak. On the 4.0, the most common leak is the valve cover in the rear. Not that big of a deal to change. You have to get your hands dirty though! LOL!
Newbie
Joined: Sep 2018
Posts: 18
Likes: 0
From: Queens, NY
Year: 1998
Model: Cherokee (XJ)
Engine: 4.0 V6
Wow! This is an old thread. Synthetic=Noisy Engine?
I have a '98 TJ with 75K. I always use synthetic Mobil1. No leaks in the 6 years I have it.
I used to have a 20 year old Volvo turbo car with 130k on it. That got Mobil1 synthetic also. No leaks, no sweating, turbo in great shape with all it's boost still available. I sold the car with a bone dry chassis.
In the jeep, I recently noticed the engine getting very noisy at cold startup. a lot of metal clacking up top. I switched to 10w40 and a bottle of Lucas. Seems quiet now.
I don't know how true it is. But my mechanic (for the stuff I can't do myself) told my years ago that storing a car with used oil will destroy an engine. Acids in the used oil will ruin seals and actually eat away the rod bearings and thin metal parts in the engine. Synthetics do not create this acid. He wasn't my oil change guy, so that wasn't his motivation. But being that I only put 5k miles on my Wrangler in the past 6 years. It sits in the garage for months on end. It sounded like the right thing to do.
I used to have a 20 year old Volvo turbo car with 130k on it. That got Mobil1 synthetic also. No leaks, no sweating, turbo in great shape with all it's boost still available. I sold the car with a bone dry chassis.
In the jeep, I recently noticed the engine getting very noisy at cold startup. a lot of metal clacking up top. I switched to 10w40 and a bottle of Lucas. Seems quiet now.
I don't know how true it is. But my mechanic (for the stuff I can't do myself) told my years ago that storing a car with used oil will destroy an engine. Acids in the used oil will ruin seals and actually eat away the rod bearings and thin metal parts in the engine. Synthetics do not create this acid. He wasn't my oil change guy, so that wasn't his motivation. But being that I only put 5k miles on my Wrangler in the past 6 years. It sits in the garage for months on end. It sounded like the right thing to do.
Newbie
Joined: Sep 2018
Posts: 6
Likes: 0
From: Texas
Year: 1997
Model: Cherokee (XJ)
Engine: 4.0
Hmm yea it’s old
Hell, for many moons if your engine wasn't using a quart between oil changes, you had to worry. I'm talking 50's here. They had things called "road draft tubes" where crankcase blowby was vented underneath the car. It took PCV valves in the 60's to change that. Now with the same usage, you have to look for a leak. On the 4.0, the most common leak is the valve cover in the rear. Not that big of a deal to change. You have to get your hands dirty though! LOL!
Newbie
Joined: Jan 2019
Posts: 1
Likes: 0
Model: Cherokee (XJ)
I heard somewhere. I want to say Trucks! or 4x4 TV on spike. Anyway, I thought I heard that in older high mileage engines it is better to run conventional oil because it is thicker and less likely to get by seals and such. On the other hand I have heard the exact opposite. Currently I am running Valvoline 75000+ High Mileage Syn. Blend. 
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