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Oil spec for 1991 4.0L?

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What does the 1991 owners manual say exactly is the correct oil for the 4.0L engine?


I bought Castrol 5W30 conventional today to do an oil change. I do not have an owners manual for my 91 but I do for my 88, and it calls for: "API Engine Oil Service Classification SF/CC. The term SF/CC must appear on the container singly or in conjuction with other designations. The SF designation alone is not sufficient." Well that was written 26 years ago so who knows if they still even have that designation on oils... anyway...


So... the Castrol oil says on the back "API Service SN, SM, SL, SJ". So now I'm not sure I should use it.


Yeah I know what you guys recommend (Rotella generally) but I'm interested in knowing what the owners manual calls for. I'm hoping someone has the manual and can check that for me. I know better than to post on BobsTheOilGuy ha ha. I'm not trying to start an oil debate.


I have case of this stuff (12 quarts) so I'm hoping I can use it...

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I run 10w30 in mine with a zinc additive, mine is a stroker with higher spring pressure, the new oils are designed for better emissions, not to protect old non roller cams.


Look up what happens when your zinc levels are too low in older motors, lots of cam failures,

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Yes, the older the rating, the higher the zinc content. Too much zinc in the oil of newer cars causes issues with emissions testing. Most diesels don't have this issue, but its starting to go away from their oils too. So keep an eye out for the rating if you care about that sort of stuff. I recently bought 11 gallons of NOS Mopar diesel oil SJ rated to keep for my MJ. That ought to hold me for a while!

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I run 10/30 in my shortbed 88, 284,000 miles. I live at 7,000 ft. in northern arizona, summertime I'll switch to 10/40 , also run some dura-lube once a year. Castrol is the only oil used in my 88, l never asked the p.o. of my 89 what oil he used, I run Castrol in it also. And Wix filters. I would think 5/30 would be a must in Canada.

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I know my owner's manual exists... somewhere. I took it out of my glove box to make a digital copy, left it sitting beside the scanner at my parents place while I was home over a year ago and never got around to scanning it before I left. My parents moved houses before I was able to make it home again, and now no one is quite sure where it went. :(


Right now I'm running 20w50 oil in the MJ... course, that's just to allow me to move it around the yard without further damage to the motor. The motor's FUBAR... knocking-from-the-bottom-end-oil-pan-full-of-glitter kind of fubar.
I had been running 5w30 before then, and while my oil pressure had always been low, it was slowly getting worse. I was going to switch to a 40 but then had that bad news before I got it done.


I'm assuming you've got an oil pressure gauge. If so, keep an eye on it. I'm told the needle should be pretty close to vertical. Mine never was, but it was within the spec that someone on here quoted. Don't remember what that spec was, though.

As far as oil weights go, (as I'm sure your know) the second number is once it's up to operating temperature, and the first number is for cold starts. A 5w30 is effectively the same as a 10w30. Based on what I know now, if I could go back, I'd be running 5w40 for the winter, probably switching out to a 15w40 for the summer. Of course, my motor has twice the miles that yours does. Supposedly it was rebuilt at ~320,000, but judging from the crap that I've put up with, I'm pretty positive the guy who did it would have reused the original bearings and mostly just put in a bunch of el-cheapo gaskets.



If you did put in the 5w30, I wouldn't be in too much of a rush to switch it out. Buddy of mine runs 5w30 year-round in his '96 4.0 XJ with no issues and good oil pressure, similar mileage to your '91. But it wouldn't hurt to run a heavier oil starting from the next time you change it.

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  • 3 weeks later...

Miraculously my owner's manual just turned up... on my teenage sister's bookshelf of all places.

Bit late, but the oil spec it gives is API SERVICE SG or SG/CD.

FWIW it also says 10W-30 for a temperature range above 0F/-18C and for 5W-30 below 60F/16C.

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