hyperbolical

Hello
I've owned my 1998 XJ 4.0 for 3 years but I feel I am still just learning about how jeeps work (I spent many years in Europe working where I did not own a car). My '98 jeep now has 105k miles and has been running fine I think but in summer if I am stuck in traffic it will usually overheat. That is the main problem-- the other one is that the gauges stop working for days or weeks at a time then inexplicably begin working again. If anyone has fixes for either one of these please let me know!!
I've been reading threads on various forums until my head is swimming about what is the best oil for older xj's in these environmental conditions. I don't have to worry about the cold weather but summer is a different story so I'm trying to be prepared.
Many suggest Rotella T6 5w40 for jeeps in hot summers such as we have here so I was all set to get some until I discovered that almost everyone in Miami is out of stock due to supply chain issues. Apparently they do not intend to have more until the 4th quarter this year. I want to change my oil this week while I am off work since I did not change it during the pandemic.
So, do I make the trip 40 minutes away to pick up the last 2.5 gal jug of Rotella in town? or will Castrol Edge High Mileage 10w30 (5 minutes away) be a worthy contender? Is there some other oil that will be better at cleaning out the engine and be great for the extreme heat here? I will use the Mopar OEM filter, as I have been trying to restore the jeep bit by bit to the original equipment.
I also have a few general questions about the oil discussions, perhaps a kind soul can answer them and help educate me.
Why is Rotella t6 a good oil for this engine even though that oil is heavy duty diesel oil? All the automatic checkers on the auto websites that tell you whether the oil or part fits your vehicle tell me that this oil is not specced for my jeep, yet many if not most posters swear by it. I understand the minimum spec for the xj is 10w30 but those same website checkers have told me a 0w20 oil fits my vehicle!! Should I just ignore the website checkers?
Is Rotella T6 15w40 ok for this vehicle as that weight seems to be more available? My jeep never turns over on the first try, I have to prime it, wait a few seconds, and it will turn over on the second try.
What is meant by Pennzoil Yellow Bottle that many others swear by? Since all the Pennzoil oils have yellow bottles now? does it refer to the original conventional oil?
Totally confused in Miami.

BruceB

I live in central Florida and have used 10W30 oil for the last 25 years. 5 1/2 quarts of oil, 1/2 quart of Marvel Mystery Oil and Mopar Filter. Never have had to go inside the motor for anything. 218000 miles...oil and filter change every 3000miles... for what it is worth....

hyperbolical

Thanks for the reply! May I ask what brand and whether synthetic, conventional or blend? I'm leaning toward 10w30 too, with the MMO and Mopar filter. Just need to settle on what type of oil.

robsjeep

5W is really too thin for Florida, even in winter. You know how thin a 5w is at room temp? It aint much thicker than bull urine. This means very thin oil on start up when your motor is cold. The jeep is not designed for such thin oils, unlike like some modern engines. Id especially not effect a change in thinner oil with out a known good working, calibrated oil pressure gage, so you can at least shut her down when pressure drops below that magic engineering number of 10 psi minimum per 1000 rpm..

10W 30 is specified under normal conditions. Unless it is Arctic condition, snowed in, Id not use any less than a 10W oil in the jeep. I run 10W 30. occasional freezing morning, and frequent 90F daytime highs in the desert I roam, even 100F + under high load wheeling, 10W 30 works fine in my XJ.

Why do you want such thin oil 5W for Florida? Dont do it, it will result in more cold start wear is my thought. save that thin stuff for your new wiz bang, overhead cam, high reving little techno engines that are designed for super thin sewing machine oil.

Want to really wreck your older rigs, run some modern synthetic 0W-10 "fuel saving" motor oil on a hot desert trip, pulling a long grade. Your pressure will go to nil quick.
Would you run 10w-40 in your differential? No, it is not specified for it. Sure it is better than no oil, but it is wrong.

. Old jeep motor is not designed for this thin oily stuff, our engines need some beef in the oil, some cushion for the pushing in them bearings.

Thats my stubborn old fert opinion for whats it worth, which is just your time to read this, thats all.


I run the K and N oil filter on my 1990 XJ, early style filter mount. HP 2007 is the model, very well made filter. I have personnel seen them dissected, much like that cat in high school biology class, where you can see the inside. If you got the later XJ horizontally stylized filter mount, then you need a different model filter, as the HP 2007 only fits the early XJ with the verticalized mount oil filter.

Have fun jeepin!

robsjeep

As a man of science, I can say with high certainty that it is a scientific FACT that switching to thicker oil will NOT...repeat...Will NOT fix your overheating problem.

Id look at the engine driven fan clutch, make sure it locks up when hot, if not then at slow travel speed you will not get enough air thru the radiator.
Run a pressure test on the cooling system, inspect it, take a look at the plugs to assure good burn is happening. make sure exhaust is not clogged, broken catalyst guts can stuff up the exhaust (I had that happen).
Also make sure you dont have too much antifreeze in the system, no more than 50% for sunny Florida. excess antifreeze reduces cooling ability of the mix.

lawsoncl

I don't think you understand how the multi-weight oil notation works. A 5W-30 acts like a straight 5W oil at 0*C, and a straight 40W oil at 100*C. A05W oil at 0* is still way thicker than the 40W at engine temp. You actually want a "thinner" oil at startup so it pumps faster. A 10W-30 might give you better oil pressure than a 5W-30 at startup on the gauge, but that doesn't translate to better pressure at the downstream at the bearings if you're pumping a thicker oil.

Actual viscosity numbers if you you want to compare for yourself.
https://wiki.anton-paar.com/us-en/engine-oil/

robsjeep

I understand exactly how she works, and zero degrees is not a likely thing in Florida. High heat is, and too thin an oil is bad. I wonder why jeep recommended no less than 10w for Florida like climate? Too thin is too thin,and that is what is happening at lower temperatures with that 5W oil stuff.
Understand, you may run what ever you want, and maybe it works ok for you, I say what I find works best for me given my understanding.

lawsoncl

You are correct that in Florida, 10W-30 is the recommended oil. See below for the chart from 2001 FSM. Your statements about room temp and 0W-30 dropping your pressure to nil implied you were one of the people don't understand the specs, and somehow think 10W-30 is thinner at room temp than operating temp. Lots of people are also onvinced they need to run 20W-50 in the summer too, under the mistaken notion that their oil temps are significantly higher in the summer than winter.

BruceB

I started with Conventional Quaker State for High Mileage engines and switched to Vavoline Synthetic for High Mileage engines about 10 or so years back.

MaskedMallard

I’d figure out your overheat problem, before worrying what oil to use.
- have you flushed & back flushed your radiator & heater core?
- might want to change the water pump, maybe the impeller is worn?
- is the thermostat lazy?
- mechanical & electric fan working as they should?
- if the radiator & engine is cold, maybe spray water from inside the engine bay to the front of the XJ, to get bugs & other such debris out of the radiator.

What I recommend, at a minimum is;
- 2 or 3 row radiator (I went a 3 row).
- high flow water pump (www.hesco.us).
- high flow thermostat housing (www.hesco.us).
- new thermostat (www.hesco.us).
- new water pump bypass tube.
- new radiator hoses & heater hoses (I installed silicone hoses).
- water wetter.
- quality mechanical fan clutch.
- make sure your fan shrouds are in place & your electric fan is working.
- use quality Mopar sensors when replacing sensors.
- at this point, might as well replace the serpentine belt.

My XJ was cooling fine. By doing the mentioned above, I’d guess I dropped operating temperature by about 5*F, according to the temperature gauge in the dash.

hyperbolical

MaskedMallard
Thank you very much for the detailed checklist. I have done alot of those things and have much less of a problem then before. But when I am stuck in Miami traffic in the middle of summer for any length of time with the AC on (and even when off) it still does overheat sometimes. I replaced the thermostat, radiator, radiator hoses and some of the sensors. I use Mopar whenever available. I flushed the radiator with Prestone and clear several times. Have checked the fan multiple times. I will go over this list again. However I need to change the oil now since I didn't do anything since Covid started and this is the only week I have off before work starts in again.
Just found a thread that said check the EGR valve. I will do that as well. that thread is here https://www.cherokeeforum.com/f2/ove...236023/index6/
Wondering if anyone else found the EGR valve to be a problem?

hyperbolical

Hi robsjeep
Yes, I didn't think it would. But I am looking for the best oil in very hot conditions, with stop-n-go traffic. Do the specs mean that 10w30 is the best oil for average conditions? ie finding the mean between Minneapolis weather vs Miami weather? or is it a fixed result based on the engine itself? With a high mileage vehicle in city traffic, 95 degrees F and up, I am wondering if a thicker oil for higher temps wouldn't be a wise thing? If its not better, why do so many people on the various forums swear by the Rotella T6? I understand it gave back great UOA at some point, whereas Mobil 1 had poor results. So there must be differences between oils. I was thinking of switching to full synthetic since I could go maybe 5k between changes.

Thanks for the comments on the overheating issue, I will check those things.

hyperbolical

Hi lawsoncl, thanks for the reply. I'm not sure I understand the last sentence. "Lots of people are also onvinced they need to run 20W-50 in the summer too, under the mistaken notion that their oil temps are significantly higher in the summer than winter."
Are you saying the engine temperature is basically the same on a very hot summer day as on a regular day in Florida? Everything seems so much hotter. The best argument for the Rotella seemed to be the stability at extreme temps plus the great UOA results. But I also heard you get lower gas mileage.
Let's say I go with 10w30. Would Castrol Edge High Mileage be a good choice?

00t444e

15w40 diesel motor oil is all I use in mine, works fine in the summer and winter. However that has nothing to do with your overheating problem. You need to do a coolant flaus and make sure your cooling system is working properly.

00t444e

Do you realize that 0w40, 5w40, 10w40, 15w40 or even straight 40 weight are all the same viscosity once up to operating temp? Only difference is the multi weight oils flow better when cold.