Gear oil or ATF for transfer case?

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I have a 1973 Chevy truck with the NP205 gear driven transfer case. My original 1973 manual says to fill it with 80w gear oil. Later manuals for the same transfer case specify ATF instead, I'm guessing this was for gas mileage reasons. The newer 205 transfer case that calls for ATF is the same as far as I know to the old one that I have.

I'd like to switch my TC to synthetic ATF for mileage reasons...but do you think I should? Is ATF going to provide enough protection? I believe it should...but I don't want to be buying a new transfer case either....

I've asked this question on Chevy forums and got both answers...
 
I'd use gear lube as recommended.
You can always use a synthetic 75W-90 for improved fuel economy.
This is still a much thicker oil than is any ATF, but it should protect this old thing as ATF might not.
Someone who knows a whole lot more about this tranfer case than I do may beg to differ with me, but I don't think you'd go wrong with a multi grade syn gear lube, while I'm not so sure about ATF.
 
Gear oil for me in a 73 transfer case. That is what the manual calls for and that is what she will get. I highly doubt that fluid will impact MPG in that vehicle.
 
Originally Posted By: toneydoc
Gear oil for me in a 73 transfer case. That is what the manual calls for and that is what she will get. I highly doubt that fluid will impact MPG in that vehicle.


X2.

I wheel a lot. Belong to many clubs, and wheel some pretty tough trails. While I know that GM spec'd the 205 for ATF(around 1980 or so), everyone I have ever met has run gear oil. It's a gear driven t-case that weighs 150 lbs.
 
I don't think it's a good idea to mix the two fluids.. Gear oil being as viscous as it is, is not going to completely drain from the case without a series of drain / fills-- You'll end up with a mix of ATF and small amount of gear oil.
 
I would use your favorite brand of GL5 gear oil. I have also heard a xw-40 HM or HDEO motor oil is a good choice also when ATF is spec'd.

transfer cases and manual transmissions that get ATF are to simplify the production line from what I have read. not sure if it is true or not.
 
I should mention I rarely use/never abuse the transfer case, no serious offroading or anything like that. Most of the use it gets is just from me trying to lube it up....

I know the fluid won't make a huge difference in gas mileage here, but I'm changing everything I can in hopes that 1/10th of a MPG adds up.

One thing I notice is the transfer case gets quite warm even on a short drive, with the thick gear oil inside.
 
I've got some era-appropriate gear oil for your truck. 3 qts. It's yours if you cover shipping.

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Originally Posted By: meangreen01
I've got some era-appropriate gear oil for your truck. 3 qts. It's yours if you cover shipping.


LOL, it still comes in the same bottle!
smile.gif
Thanks for the offer but I'd like to use something a little more modern.
laugh.gif
Lots of guys like to keep their trucks original though, they'd probably want that!
 
Originally Posted By: 73Chevy
Originally Posted By: meangreen01
I've got some era-appropriate gear oil for your truck. 3 qts. It's yours if you cover shipping.


LOL, it still comes in the same bottle!
smile.gif
Thanks for the offer but I'd like to use something a little more modern.
laugh.gif
Lots of guys like to keep their trucks original though, they'd probably want that!


Darn. Well how about I ship em to you and I eat the shipping? Things are getting dicey on my end and the wife says to get em off the stove, Lol.
 
Originally Posted By: meangreen01
Darn. Well how about I ship em to you and I eat the shipping? Things are getting dicey on my end and the wife says to get em off the stove, Lol.


If you wanna ship 'em, I'd be happy to add them to the oil collection.
smile.gif
I'm not crazy about keeping my truck totally original but I do like to keep all the old original parts on hand.

I'll send you a PM.
 
IMO...you can go to AZ and get some Mobil 1, full synthetic, gear lube 75w-90 on sale for $6.99 a quart.

Normal price is $10.99 for this M1 gear lube so your ahead of the game. I would not use ATF as a replacement for gear lube.


Originally Posted By: 73Chevy
Originally Posted By: meangreen01
I've got some era-appropriate gear oil for your truck. 3 qts. It's yours if you cover shipping.


LOL, it still comes in the same bottle!
smile.gif
Thanks for the offer but I'd like to use something a little more modern.
laugh.gif
Lots of guys like to keep their trucks original though, they'd probably want that!
 
So, how come nobody thinks ATF is good enough? I mean...transmissions are full of gears and bearings just like my transfer case, right?

There's obviously a huge difference in viscosity, which I think will translate into less heat in the TC and therefore less gas used.

FWIW, I've heard to use whatever fluid you're using in your transmission in case the seal between the two starts to leak. ATF behind an automatic and gear oil behind a manual.
 
Personally, I just don't feel comfortable running ATF in the long run for a gear driven, cast iron, T-case. Typically, the T-cases that do specify ATF, have pumps and planetary gear sets for low range.

I have both types in my stable, and the gear driven get 80W90 and the chain drive get ATF. I'm sure if you ran ATF, you would be OK, as the 205 is a stout case. I would research to see if Ford ever spec'd ATF, as they used the NP205 too.

Any seal breaches I have heard of the tranny empties into the t-case. By that time, you would know something is wrong as the tranny would most likely slip. I suppose having the same fluid would be a good thing just in case though.

If you want to run ATF, I recommend one in synthetic flavor. I run Mobil 1 multi-vehicle ATF in my trail rigs and a couple of drivers that run New Process aluminum T-cases. I always like the piece of mind knowing that if I flat towed in the winter, I'd still have instant lube circulating. It can take the heat too.

If you decide to run ATF, please give some feedback on noise, heat, etc. It would be interesting to see the results.
 
Originally Posted By: Zaedock

...the gear driven get 80W90 and the chain drive get ATF.


Ding-ding-ding WINNER!
 
Originally Posted By: 92saturnsl2
I don't think it's a good idea to mix the two fluids.. Gear oil being as viscous as it is, is not going to completely drain from the case without a series of drain / fills-- You'll end up with a mix of ATF and small amount of gear oil.


We had a Mazda 626 a number of years ago where the owner's manual spec'd some kind of gear oil (I don't remember what kind, I'm guessing some kind of GL-4) for the manual transmission, so I bought the required number of quarts, put it up on the lift at school, and what drained out was nice and red. The guy who ran the shop said it looked like Dexron to him, so I called the local Mazda dealer. Sure enough, the service guy told me some of those transmissions had gear oil, some ATF- and to never mix & match. He said if you drain ATF, replace with ATF, same for gear oil. So I returned the oil and swapped it for Dexron, no problems thereafter.
 
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