What oil do you guys use on your 4.0s?

Kenneth G Zinis

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Nov 6, 2018
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Bethlehem, PA
In PA see temps all over the map. POs used to use Mobil 1, so I used syns for a while. Had a little weeping from rear main seal. Started using conventional 10-30, rear seal is now dry and Wrangler tap is only intermittent. If you can get high Zn oil like Valvoline VR oils.
 

Jerry Bransford

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Nov 9, 2015
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Fleming Island Florida
In PA see temps all over the map. POs used to use Mobil 1, so I used syns for a while. Had a little weeping from rear main seal. Started using conventional 10-30, rear seal is now dry and Wrangler tap is only intermittent. If you can get high Zn oil like Valvoline VR oils.
Good decision on the conventional 10W-30, that can save the seals from starting to leak But on a stock TJ engine I would NOT waste my $$$ or time adding more zinc to what's already in the engine oil. Yes the industry was forced to reduce how much zinc is present to prevent damaging the catalytic converter but it's not so low that it causes engine damage to a stock TJ engine. There's not even pressure on the cam lobes from the factory valve springs to cause damage as evidenced by the severe lack of complaints of valvetrain damage in our TJ engines. If I was running a high-performance race engine with a high lift cam and stiffer valve springs I'd definitely be adding additional zinc but I'm not so I don't.
 

Pantera99

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Sep 2, 2022
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Location
Gilbert, AZ
Good decision on the conventional 10W-30, that can save the seals from starting to leak But on a stock TJ engine I would NOT waste my $$$ or time adding more zinc to what's already in the engine oil. Yes the industry was forced to reduce how much zinc is present to prevent damaging the catalytic converter but it's not so low that it causes engine damage to a stock TJ engine. There's not even pressure on the cam lobes from the factory valve springs to cause damage as evidenced by the severe lack of complaints of valvetrain damage in our TJ engines. If I was running a high-performance race engine with a high lift cam and stiffer valve springs I'd definitely be adding additional zinc but I'm not so I don't.

Hmmm, I usually run Valvoline Maxlife & seems to have helped the rear main leak. Was gonna try Castrol HM but I see its semi synthetic too. Maybe I'll just try regular conventional. Was thinking of checking the valve pan cover bolts too because there's alot of gunk towards the back of the valve pan.
 
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