# What is the correct differential gear oil?



## HickBoy (Aug 5, 2018)

I have a 1965 GTO and I believe (I am still green at this so I may be wrong) the original differential. The owners manual states to to use SAE-80 or SAE-90 Multi-Purpose gear lubricant meeting MIL-L-2105B requirements. It also states that if you have a Safe-T-Track you should get Lubricant Part No. 1050081. So I have no idea if I have a Safe-T-Track but I listed the specs that I can see from the outside.

My questions for a real newbie like myself (who totally enjoys trying new things...): 

1. What differential oil should I be using?
From reviewing the forum it seems something like Lucas 80W-90 GEAR OIL or any 80w-90 Gear oil would be acceptable.

2. Would attempting a differential oil change be something that I can royally screw up?
From all of the videos I have seen online it appears that making sure I properly seal the cover with a paste gasket maker and torquing the bolts correctly is my biggest hurdle.

Specs:

10 Bolt Cover
Casting Code: 9779822
Gear Code: WF

I believe that equates to a 1965 GTO Differential with 3.23 gears and an Automatic transmission which matches what my PHS docs state.


BTW: Thank you to everyone who answers these types of questions for the folks like me who are learning/breaking things as we go. I am totally enjoying the goat!


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## PontiacJim (Dec 29, 2012)

You do not have the Safe-T-Track (positraction) rear end.

Yes, any 80W-90 gear oil will work. I would not use a synthetic. Lucas brand is a good choice and what I have used.

Before pulling off the rear differential cover, I would first make sure that you can remove the fill plug. Typically the square hole in the plug is the same as a 3/8" drive breaker bar or ratchet. You don't want to find out that you cannot back out the fill plug for some reason and then have a differential that you drained the fluid out of and now cannot add any. If you can't get the plug loose, or it is more than your skill level, you can then drive it to a shop and have them do it - protects you from getting stuck in a bind.

If you can get the plug loose, then go ahead and drain the differential. Scrape all old gasket material off with a putty knife or razor blade so you have a nice clean surface. You can spray the cover with Brake Clean to get all the gear oil off and spray some on a rag and wipe the machined surface on the rear-end housing to get the gear oil off it.

Not sure what everyone uses on the gasket as far as sealer goes. I like the 3M Weatherstrip and Gasket Adhesive. Most will know it by its yellow color, but they also make it in black. I also like the Indian Shellac which is like a molasses you apply with a brush in a thin layer. Then there are the assorted silicone gasket materials. You can ask your local autoparts store what they might use as well. Just don't get too crazy on the thickness of your sealant, especially the silicone, as when you torque down the cover it will squeeze out - this is one of the reasons I like the Indian Shellac with brush applicator as you can brush on a nice thin even layer and know you did not over do it.

Whatever you use, coat each side of the differential cover gasket with it and install it and your cover. Then torque your bolts down.

Add your gear oil and fill just until the gear oil runs back out of the hole - make sure the car is level and not jacked up or you will not get the correct fill level. Too much gear oil and you could blow out a seal from internal pressure.

I think that about covers it. YouTube also has a couple videos if needed.


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## integrity6987 (May 10, 2017)

HickBoy - super clean under there!!

ALL: How often should this be done as preventive? Mine has the tag and I know it is Safe-T-Track from PHS (and observation ;-D ) On my car this would have been done in 2004 - but only about 13K miles ago.

In my mind, this should not need preventive maintenance - if not leaking. Not sure if PMD ever recommended anything.


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## PontiacJim (Dec 29, 2012)

integrity6987 said:


> HickBoy - super clean under there!!
> 
> ALL: How often should this be done as preventive? Mine has the tag and I know it is Safe-T-Track from PHS (and observation ;-D ) On my car this would have been done in 2004 - but only about 13K miles ago.
> 
> In my mind, this should not need preventive maintenance - if not leaking. Not sure if PMD ever recommended anything.




The factory manual says "Change lubricant only when necessary to disassemble." I have never changed out rear axle oil in any of my cars. For the manual transmission fluid says the same thing.

The more important thing would be to once and a while check the fill level on the differential and the manual transmission.

I guess like other things, some of us simply feel we need to flush out the rear differential or manual transmission as a preventative measure regardless of what the factory says. Can't see it hurting anything. I suppose with a posi, like an automatic trans, if you drain and refill with fresh fluid it could flush out all the small clutch particles that may be the only thing keeping or helping the friction clutches from slipping if they are just on the verge of wearing completely out.

I just did a gear oil flush & fill on my brother's 1948 International truck with 41,000 miles. Not a lot of miles per say, but it does have a lot of years. Non-synchro transmission so it has to be double clutched or you scrape gears. What came out did not look too good and I did have small metal shards drain out with the oil - from someone scraping gears. So I would say in this instance, changing out the gear oil was indeed needed. But keep in mind that the older gear oils were not as good as today's formulated gear oils, but flushing does get out any metal that could be splashed around with the oil and get into bearings. The '48 manual says to change rear differential fluid every 10,000 miles, but says nothing for the transmission. I plan on flushing the differential and see what pours out of it as well.

So, it may not be necessary to ever flush and fill a differential or manual trans unless you have to pull it apart for some major repair - according to the factory. My 2015 Hyundai manual says my trans fluid should not need changing in its lifetime unless severe driving conditions exist. Define "severe conditions" & "lifetime." I just did a flush at 90,000 miles because it is cheaper than a rebuilt/new transmission, and lifetime in my book is 300,000 plus and somehow I just don't feel the original trans fluid was meant to go 300,000 miles plus without a change. So maybe it simply boils down to "peace of mind" in knowing the gear oils are fresh while the car is in your ownership and you won't have to do it again in your lifetime. :thumbsup:


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## nardulli (Sep 13, 2009)

Any name brand non synthetic gear oil would be fine. You won’t need the friction modifier. A THIN coat of a name brand RTV is all you need on both surfaces. The pics brought me instantly back to 1975 when I was making one good 66 GTO out of 3 wrecks. (My Dad’s way of keeping me broke and out of trouble). The task is a simple one. Oil doesn’t “wear out”. It becomes contaminated. It’s a good idea to change your differential oil for several reasons: 1. You will be able to inspect the condition of the gears and look for metal in the oil. 2. You will have peace of mind from knowing. That diff may have a magnetic drain plug. If it doesn’t it’s my practice to replace it with a magnetic plug. Great that you’re learning into this. Don’t hesitate to ask questions.


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## HickBoy (Aug 5, 2018)

Thank you all for the great info. I am ready to attempt the change:

1. Have my two bottles of Lucas 80W-90 gear oil.
2. Have two tubes (cause I know I will mess up the first time) of Permatex 81182 Gear Oil RTV Gasket Maker.
3. Have confirmed I can remove and replace the plug.
4. Purchased some 6000lb Torin Red jacks (had 3000lb) but I want to lift the car higher to make this easier.
5. I would buy a magnetic drain plug but I am not sure what size to order and worry I would strip screws.

BTW, when I stuck my finger in the oil it was mid to dark brown almost like a clean motor oil right when it starts to get dirty and it was about 1/4 in below the hole. Even if it does not require changing, I want to give it a whirl to verify I can do it.
Last but not least I have some great advice on the forum. Thank you again!


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## PontiacJim (Dec 29, 2012)

HickBoy said:


> Thank you all for the great info. I am ready to attempt the change:
> 
> 1. Have my two bottles of Lucas 80W-90 gear oil.
> 2. Have two tubes (cause I know I will mess up the first time) of Permatex 81182 Gear Oil RTV Gasket Maker.
> ...




Excellent! Go for it, sounds like you have all your ducks in a row. There is no drain plug on these rear ends, just the fill plug - which could be magnetic. That is why it can become a messy job as you have to pull the back cover to drain it. Here is a YouTube video that is pretty good and to the point.


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