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15W40 too thick for 4.0L???

23K views 24 replies 22 participants last post by  1990JEEPXJ 
#1 ·
I was recommended by a knowledgeable source to use 15W40 for oil in the 4.0L. Is this too thick or is it a good amount to use for hard, hot summer driving.

Lately the engine has been running hotter. The coolant is nasty and in desperate need of a change, I am assuming that is the cause, but could the oil be causing the engine heat as well?
 
#3 ·
The 15w40 is fine, but it won't fix the overheating. You need to flush the cooling system completely, and replace with a 50/50 mix. I would recommend replacing everything in the cooling system that is due as well. A new radiator cap and thermostat, along with hoses, waterpump, and fanclutch if it has been five years or more.
 
#7 ·
With the next oil change going into the winter, would 15/40 still be fine for winter? or should you stick with a 10/40 even in winter?
 
#11 ·
i run 15w40 in my hi time engines due to the xtra viscosity helps cushion and take up bearing clearance caused by wear, or whatever, and for really noisy {ford } engines, i use 20 w 50. The major airline I used to work for used 15w40 penzoil in all of their ramp vehicles with no ill effects, even on brand new equipment.
 
#12 ·
If you live in sub zero climates, use a 5-30
If you live in the hot desert, a 15-40
If you have 200,000 miles and a hot climate, a 20-50
If you have 200,000 miles and a cold climate a 10-40

A 10-40 is a good combination blend for most average mileage motors in most of the United States.
A 5-30 is a good oil for most new motors or low mileage motors in most of the US.

Be sure to check your manual if you have a new or newer vehicle for their recommendation. There are occasional exceptions.
 
#22 ·
hmmm. good overall info, but my 96' owners glovebox manual says not to use 10-40.

me? cheap api-rated ("starburst symbol") 5w30 every 5-6k miles and bigger PH8 filter instead of the stock, tiny, ph16.

as for your oil choice: as long as you make 13psi at idle (from the book) and more than that during driving...dino, synth, blend, straight weight 50 oil, is your choice.

in any case, it seems that older, rated, oils have more ZDP (zinc dialkyl phosphate) that flat tappet lifters like than newer rated oils. Also, any "diesel" rated oil has a good amount of additives as diesels place much higher stress on oil than gasoline engines (they have 20:1) compression ratios, trucks go 10k on oil changes or more, etc).

as that, chevron "delo" oils really rate well for such use.

old-school multi weights use to rate the first number for lowest expected temp when starting motor.... example 10w-30 meant 10'F ambient air temp is the lowest temp before the motor oil is a glob of jelly in the crankcase.

in north carolina, i used 20w-50 in old plymouth TC3 "smoker" that burned a quart every 100-300 miles (defect factory valve stem oil seals), when it was 5'F in the morning at times, without any motor wearing out issues.

you can go the other way with 0-50w synth oils, or even 5-20 or 0-20 weight oils....personally, that's like putting kool-aid in your crankcase, too thin for my flavor of engine DD abuse.

so: go with whatever keeps it quiet with sufficient oil pressure at idle/cruise rpm's and be happy. as for change intervals... I have yet to see any engine changed at 3k religiously outlive the 5k interval motor. Each motor was still good and living long after everything bolted to each engine gave up a ghost or two!
 
#17 ·
Most 15w40 oils off the shelf have a detergent in it intended for diesel engines. I've never seen a 15w40 bottle that doesn't "for diesels" on it.
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Exactly. :) Diesel engine oils are a really good choice today for an older-tech flat-tappet engine like the 4.0.

If you read the whole label, they also carry the API SL or SM gasoline engine ratings in addition to the diesel CI-4 or CJ-4 rating. They won't hurt anything.
 
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#14 ·
at what point are you sacrificing Pressure with flow? If I am thinking of my fluid dynamics class correctly, a thicker fluid at 40 psi will move slower then a thinner fluid. so in general don't you want to go with the thinnest oil your motor can take? (i.e. if it does not have any valve train noise stay with OEM rec., other wise try going up in thickness until it is solved.) It seams like there are a lot of Motors out there running on a thicker oil when there is no need for it.
 
#15 ·
Magus, i agree with you but i think the bottom line is to keep oil in it and keep it changed and use good filters.
I usually run 5w30 in my newer cars and 10w30 in my older cars but mine don't have valve train noise.
 
#16 ·
15w40 is fine- the 4.0 seems to like heavier grade oils. If you don't mind spending for it (its not all that much more, really) a synthetic 5w40 like Shell Rotella T synthetic will give you the same heat protection but will get flowing quicker when you start it up cold. But lots of people run regular Rotella T 15w40, Chevron Delo, Valvoline Premium Blue, or other diesel engine oils. The 4.0 works great with them.

High mileage oils like Valvoline Max Life are also good in the 4.0 if you want to stick with the recommended 10w30 grade.

But either way, fix the cooling system- the oil has nothing to do with overheating.
 
#18 ·
Cooling Issues?

So for the cooling system, the recommened changes would be water pump, fan clutch, radiator cap, and a coolant flush/fill? What about the thermostat? Do that with the water pump?

I am trying to figure out if my water pump is bad-I think it is.

I also think the fan clutch is done too, because whenever I turn the A/C on, the electronic fan engages and it makes this weird whistling/humming noise.
 
#19 ·
The local independent parts shop really talks up the Rotella T as well for muscle cars that like it better for better lubrication - ran it in a little tow motor engine, and thinking about it in my current XJ as well - not much more and has some benefits for the great white nord.
 
#20 ·
Okay im running a 97 4.0 in a 94, new gaskets all the way around 150k,had a little lifter rap Flushed her with 1 qt trans fluid into the oil for 20 mins, drained refilled with 10w30 royal purple fulll syn+ stp treatment, Have 1000 miles mostly off road and high revs, with no leaks no wraps, I hit the mud boggs this past sunday and hit 6000 rpms for a good 40 seconds, and nothing out of the motor not even a cough, oil still looks awesome kinda purple clear color, I blew my front spider gears 100 yards in though,think it was the 37's n 3.55s I wold highly recommend ROyal purps for any applications as long as ur not leaking, Stp for any noise
 
#21 ·
15w40 will work fine in the 4.0L. In my opinion it's on the thick side for winters in New York. 4.0Ls do tend to like thicker oils. I personally run Valvoline MaxLife 10w-30 because that's a thicker 30wt oil. The SM zinc levels in 4.0L engines is way overrated. I did a UOA with MaxLife that showed excellent wear numbers, which wouldn't be the case if the camshaft was being damaged. In short, I still feel like a thicker 5w30 or 10w30 is the best way to go for most people.
 
#25 ·
it will hold the required pressure for the cooling system to operate correctly. if it doesnt hold the required pressure, coolant will escape into the overflow and bubble over. the loss of the coolant and the lack of pressure in the cooling system will cause it to overheat.


and i run either 15w40 shell rotella or chevron delo, or i run 20w50 castrol high mileage. both of which i use the lucas oil additive.
 
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