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Old 04-13-2014, 09:13 PM   #1
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Fluid Type Confirmation:

Specs: 1991 2.5L / Dana 30 / Ax-5 / NP231 / Dana 35

Can someone please verify what type of fluid is needed in the Transmission and Transfer-Case? I keep reading that some guys put Gear Oil in the T-Case and Some Put ATF. Also I seen that the Tranny can take Gear Lube or ATF. Which is it and why? So my real question is, what does the Manufacturer say and why do people decide to use another type of oil?

Also, I'm changing the Differential Oils too. Is it ok to add a half-bottle of Lucas to each Dana 30 and Dana 35? Or will that make it too thick? Going to use GL 75-90w for each so will that work with a Lucas mixture? Or should I just use 80-90w and forget the Lucas? Thanks in advance.

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Old 04-13-2014, 10:26 PM   #2
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The 1991 Factory Service Manual states GL 75-90w gear oil for both differentials & 5sp transmissions. ATF in the Transfer case.
Some people have started using 10-40 motor oil in the transmission and claim smoother shifting. While I believe you can get away with this in a daily driver, if you plan to do any towing or extreme duty I recommend sticking with gear oil in the Transmission.

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Old 04-13-2014, 11:02 PM   #3
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Quote:
Originally Posted by Defcon 1 View Post
Specs: 1991 2.5L / Dana 30 / Ax-5 / NP231 / Dana 35 Can someone please verify what type of fluid is needed in the Transmission and Transfer-Case? I keep reading that some guys put Gear Oil in the T-Case and Some Put ATF. Also I seen that the Tranny can take Gear Lube or ATF. Which is it and why? So my real question is, what does the Manufacturer say and why do people decide to use another type of oil? Also, I'm changing the Differential Oils too. Is it ok to add a half-bottle of Lucas to each Dana 30 and Dana 35? Or will that make it too thick? Going to use GL 75-90w for each so will that work with a Lucas mixture? Or should I just use 80-90w and forget the Lucas? Thanks in advance.
Transmission spec from the factory is now 10w30 motor oil. Go in and ask for tranny oil and that is what they will give you.
Transfercase is ATF.

Lucas is snake oil and is garbage.
Proof is in the display at the parts store. Mix it up for about a minute. And look at all the air bubbles. Mix up the normal oil and notice the lack of air bubbles.
Air bubbles are bad. Air gets hotter than the oil and gets hotter faster. This causes excess heat to form inside the engine or diff or whatever you are putting it in. This is obviously not good. Stay away from Lucas unless you need air tool oil.
Causes faster wear on your parts.
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Old 04-14-2014, 09:25 AM   #4
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Originally Posted by Defcon 1 View Post
Specs: 1991 2.5L / Dana 30 / Ax-5 / NP231 / Dana 35

Can someone please verify what type of fluid is needed in the Transmission and Transfer-Case? I keep reading that some guys put Gear Oil in the T-Case and Some Put ATF. Also I seen that the Tranny can take Gear Lube or ATF. Which is it and why? So my real question is, what does the Manufacturer say and why do people decide to use another type of oil?

Also, I'm changing the Differential Oils too. Is it ok to add a half-bottle of Lucas to each Dana 30 and Dana 35? Or will that make it too thick? Going to use GL 75-90w for each so will that work with a Lucas mixture? Or should I just use 80-90w and forget the Lucas? Thanks in advance.
This topic has been thoroughly covered, so if you search around a bit, you'll find a ton of threads on it.

Everyone has different opinions - read and make up your own mind. Mine is 10W30 for the transmission and ATF for the transfer case. I change both of them regularly, so I just use Walmart's cheap brands. I don't want the extra expense of the synthetic.
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Old 04-14-2014, 11:09 AM   #5
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I think everyone pretty much uses ATF in the transfer case, but the manual transmission is a whole different story.
I've read what seems like a thousand opinions on that subject as it relates to using GL5 or 10W30 in the manual transmission, and ultimately decided to use Synchromesh. I use my Jeep for tooling around town for the most part....no towing, a little highway, and no serious off-roading.
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Old 04-14-2014, 03:10 PM   #6
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X2 on the synchromesh. I really noticed a difference. Also, I used 80w-90 in my diffs. It's like half the cost of 75w-90 and other than the winter temperature portion of the spec it's the same thing (for non winter spec they are both SAE 90) I wasn't shelling out the extra money for the slight difference in the winter, every other vehicle I've had called out 80w-90. It's good enough. As for the transfer case, I run tranny fluid.
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Old 04-14-2014, 03:31 PM   #7
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You can use 10w30 motor oil or 75w90 gear oil in the trans. and 75w90 gear oil in the diffs. I use full synthetic in both. Full synthetic gear oils are "yellow metal" safe. The reason Chrysler went with motor oil is because certain gear oils at that time ate the yellow metals in the trans. Modern synthetic gear oils don't. They'll indicate safe for yellow metals on the bottle. I would stick with ATF in the transfer case. You can also use synthetic ATF.
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Old 04-14-2014, 05:03 PM   #8
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I've been running 75w90 in my differentials, ATF in the transfer case and 10w30 in my transmission (all full synthetics). I haven't had any issues at all. But I think I'm switching to he synchromesh for my transmission this summer.
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Old 04-14-2014, 08:37 PM   #9
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Originally Posted by StanF View Post
This topic has been thoroughly covered, so if you search around a bit, you'll find a ton of threads on it.

Everyone has different opinions - read and make up your own mind. Mine is 10W30 for the transmission and ATF for the transfer case. I change both of them regularly, so I just use Walmart's cheap brands. I don't want the extra expense of the synthetic.
I did search and all I found was people saying what they use but not why. Wrangler-Haven answered me specifically for my 1991 for what the FSM requires, that's all I needed. I was just wondering if there is any benefit to running Gear-Oil in the T-Case over ATF and why someone would switch from what the book says. Changing oil viscosity is one thing to me and that's determined on use and local-climates, but when someone says you can change over to a different fluid-type I like to know why in case things change over the years or if they had found issues with using that type/viscosity of fluid during later years of production. Thanks everyone for the replies.
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Old 04-15-2014, 12:51 AM   #10
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Atf+4 is the one I would use in T-case. Takes all the questions away.
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Old 04-15-2014, 06:56 PM   #11
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Originally Posted by Defcon 1 View Post
I did search and all I found was people saying what they use but not why. Wrangler-Haven answered me specifically for my 1991 for what the FSM requires, that's all I needed. I was just wondering if there is any benefit to running Gear-Oil in the T-Case over ATF and why someone would switch from what the book says. Changing oil viscosity is one thing to me and that's determined on use and local-climates, but when someone says you can change over to a different fluid-type I like to know why in case things change over the years or if they had found issues with using that type/viscosity of fluid during later years of production. Thanks everyone for the replies.
I've been going back and forth with a number of individuals for years whether to use motor oil/gear oil in the transfer case. I will only use ATF. I specifically use synthetic ATF. Some say using anything but ATF will burn out the pump, others say they've been running it for years without any issues. Since no one has been able to convince me of any benefit, other than personal opinion, I use what the book indicates, ATF. No problems with my 89 in 25 yrs.
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Old 04-15-2014, 11:57 PM   #12
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On the diff oil: can you use gear oil for posi-trac that has the additive in it in a non-posi rear end?
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Old 04-16-2014, 08:26 AM   #13
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On the diff oil: can you use gear oil for posi-trac that has the additive in it in a non-posi rear end?
Yes. I use full syn 75w90 in the dif. which has the additive for "limited slip". It's not a problem. I haven't seen full syn. gear oil that doesn't have the additive. Either way, there's no issue if you have an open dif.

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