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Unread 05-20-2013, 08:58 PM   #1
ctrombley
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15W-40 in my 4.0?

I have a buddy who swears that 15W-40 in my 2005 LJ is the best. I have also been doing my research and can't find anything to argue against it. Does anyone have any input on this topic?

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Unread 05-20-2013, 09:05 PM   #2
dmill89
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Do you have oil pressure issues?

If not don't run anything as thick as 15w40. 5w30, 10w30, 5w40, and 10w40 are the preferred viscosities for a 4.0L I6 in good condition, 15w40 is usually used as a "last ditch effort" on severely worn engines to squeeze a few more miles out of them. Thinner oil flows better and runs cooler, so as long as you have good pressure a thicker oil will likely do more harm than good.
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Unread 05-20-2013, 09:07 PM   #3
ctrombley
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No oil pressure issues. Everything is smooth I just don't know when the last oil change was done because I bought it about a month ago.
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Unread 05-21-2013, 10:04 AM   #4
tp72chev
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Your engine main and rod bearing tolerances are way too tight to run 15w40 weight oil. You can end up creating main bearing wear by using that oil. When in doubt use the oil recommended by the manufacturer. If your looking for added protection you can use Lucas oil stabilizer.
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Unread 05-21-2013, 10:55 AM   #5
mike_dippert
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I am currently running Mobil1 5w-40 tubro diesel oil. I have a failing lifter and research revealed it would help. It has. Only time will tell how much. I'm going to get an oil analysis in Oct when I change it out.

As a bonus, you dont need to alternate summer/winter viscosities if you get below freezing winters.
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Unread 05-21-2013, 12:19 PM   #6
Newtons3
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15w-40 wont hurt anything at all. Ran it in 6 of my 4.0 and never had a problem.
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Unread 05-21-2013, 08:39 PM   #7
tp72chev
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The other question would be why would you want to run that thick of oil. Thicker oil robs both horsepower and fuel mileage without giving any more protection. You should really search and understand oil viscosity and sheer before listing to anyone. Don't just do it because someone said it would be ok. They're not the ones buying you a new engine if or when yours fail.
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Unread 05-21-2013, 09:28 PM   #8
BIGBADWOLF
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Quote:
Originally Posted by dmill89 View Post
Do you have oil pressure issues?

If not don't run anything as thick as 15w40. 5w30, 10w30, 5w40, and 10w40 are the preferred viscosities for a 4.0L I6 in good condition, 15w40 is usually used as a "last ditch effort" on severely worn engines to squeeze a few more miles out of them. Thinner oil flows better and runs cooler, so as long as you have good pressure a thicker oil will likely do more harm than good.
I run 15w40 but it is because I have only 4psi oil pressure...been this way for over 30,000 miles. Once engine reaches 210 degrees though the viscosity isn't that much different than 10w30. My oil pressure gage reads the same with either weight oil once engine is warmed-up.
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Unread 05-22-2013, 07:47 AM   #9
cruiser54
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Quote:
Originally Posted by tp72chev View Post
Your engine main and rod bearing tolerances are way too tight to run 15w40 weight oil. You can end up creating main bearing wear by using that oil. When in doubt use the oil recommended by the manufacturer. If your looking for added protection you can use Lucas oil stabilizer.
Not true at all. The difference in 5w to 15w is negligible.

And Lucas? That stuff's like molasses!!

Read this.

http://web.archive.org/web/201009261...ucas/lucas.htm
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Unread 05-22-2013, 09:05 AM   #10
Metallic06
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If your oil pressure gauge is reading 4 psi that could be your oil pump, I'm no mechanic but don't drive it until you find out for sure
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Unread 05-22-2013, 09:37 AM   #11
Newtons3
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Quote:
Originally Posted by tp72chev View Post
The other question would be why would you want to run that thick of oil. Thicker oil robs both horsepower and fuel mileage without giving any more protection. You should really search and understand oil viscosity and sheer before listing to anyone. Don't just do it because someone said it would be ok. They're not the ones buying you a new engine if or when yours fail.
This is an absolute fact. Don't do it because someone else said to do so. Also, don't NOT do it because of someone else's fear. The wide bearing tolerances and the usage of the traditional flat-tappet, hollow pushrod, hydraulic camshaft all will safely allow 40w. In fact, since the lobe of our camshafts are basically splash lubed, an argument could be made that the thicker oil could help better lube our trouble prone lifter/cam sufaces. The very idea that a 10W change in viscosity (In this engine - context please) will cause bearing damage is strange to me. This isn't the first time I've heard that. But I've been around extreme pressure oil systems, turning many RPM's and have never experienced this. Most of the erosion we've seen has been due to overheating of the bearing surface. It has been said that the lower oil weights contribute to better efficiency and economy. While I actually do not dispute this fact, I do argue the logic. When the operaring clearances do not dictate a lower weight, generally, the current recommendations are to run the minimum acceptable weight based on the ability to eke out better fuel economy over the life of the vehicle. This will be realized over some time, not on individual tanks where I would challenge anyone to show me empirical results that most of us can justify. Once up to temperature, there will be negligible differences between the energy required to spin an engine running 30w and 40w oils. Recognizing that the priorities of the manufacturer, the government, and myself are all different, I choose the oil I use (and the change intervals) based on my priorities. Those would include protection, longevity (both the engine and the oil), and rejection/suspension of contaminates. I still run 15w-40 in my 97 and will continue to do so until the new engine is installed.
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Unread 05-22-2013, 11:29 AM   #12
BIGBADWOLF
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Quote:
Originally Posted by Metallic06 View Post
If your oil pressure gauge is reading 4 psi that could be your oil pump, I'm no mechanic but don't drive it until you find out for sure
No,that would be wishful thinking. The engine is tired and the clearances are greater. This can happen and the engine still be within factory specs, if the wear is evenly distributed in several places it would only take .001" to cause this. No knock and oil on rockers,no worry for awhile. Like I said it has read this low for over 30k. It doesn't necessarily take pressure to make an engine happy as long as oil is flowing to all moving parts. The ONLY cure is to rebuild...when she starts knocking (and who knows when that will be)I'll drop in a new engine.
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Unread 05-23-2013, 03:44 AM   #13
repairwind
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While I actually do not argument this reality, I do claim the reasoning. When the operaring clearances do not determine a reduced bodyweight, usually, the present suggestions are to run the lowest appropriate bodyweight in accordance with the capability to eke out better gas mileage over the lifestyle of the automobile.
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Unread 05-23-2013, 06:24 AM   #14
BIGBADWOLF
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Originally Posted by repairwind View Post
While I actually do not argument this reality, I do claim the reasoning. When the operaring clearances do not determine a reduced bodyweight, usually, the present suggestions are to run the lowest appropriate bodyweight in accordance with the capability to eke out better gas mileage over the lifestyle of the automobile.
Yes. The switch to thinner oil was not so much an Engineering decision as it was government pressure to increase C.A.F.E. standards...period.
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Unread 05-23-2013, 06:48 AM   #15
cruiser54
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Originally Posted by BIGBADWOLF View Post
Yes. The switch to thinner oil was not so much an Engineering decision as it was government pressure to increase C.A.F.E. standards...period.
Yep. When you see something like this, follow the money or the government.

Especially if you're told it's good for you.
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