# Rear diff fluid change.



## Hoofer814 (Apr 11, 2020)

Hello everyone. Any fluid recommendations for the rear differential on a 66 Pontiac Tempest? It has a 300 automatic transmission in it that was installed by the previous owner to mate up with the 455 engine he installed. And will i need the friction additive? Thanks!


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## Jim K (Nov 17, 2020)

You'll only need the friction additive if you have a Safe-T-Track (Posi) rear end. Read the label for the brand/type of oil you buy. Some will include a form of friction modifier in their oil and label it for limited slip differentials. I've had good luck with either Lucas oil and Brad Penn 80-90 and use friction modifier. I use GL4 because i can use it in my rear end as well as my 4 sp. GL5 is not for manual transmissions. From 1966 they may have used a GL3 from the factory but I'm pretty sure you can't find that any more.
this link may be of some help to you.. Others may chime in that can give more definition around the different grades.
Gl4 Vs. Gl5: What Is The Difference? | Rx Mechanic


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## Jim K (Nov 17, 2020)

I almost forgot.. For posi trac rears, you almost must run friction modifier. Some folks will run it even if they don't have a locking rear end as the belief is it will extend the life of the gears..


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## Baaad65 (Aug 29, 2019)

Strange Engineering recommends Lucas 85-140 conventional that what I used in my 12bolt and install a magnetic fill plug...cheap insurance.


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## Greek64GTO (Dec 13, 2015)

The 10 bolt and the 12 bolt both call for 75w-90, let's call it GL-5 as a modern upgrade. As was noted by Baaad65 if you want insurance (presume protection) you can call for a 75w-140 or a 85w-140. That is the same viscosity I use in my diffy on F350 Super Duty diesel tow rig. 

Now imagine using a 50 weight oil in your engine and pushing the oil pump to increased load and resistance. The same thing in your 10 or 12 bolt or 9 inch. I truly don't think you need to be using a #140 weight gear fluid.

Now imagine a pie chart from 9 o'clock to 12 o'clock. That is the area of viscosity and protection of the 75w-90 gear lube. Now extend your chart from 12 o'clock to 3 o'clock, that is the area of pie chart we will assign to 75w-140. This is just a simple way of explaining a range that has been assigned by SAE lube engineers. 

In order for a lube quality as a 75w-90 it must come in somewhere between 9 - 12. It can come in right on the line of 9 and and still be satisifying this standard. Some brand new lubes don't even make the grade and that is truly a shame. It takes time to weed them out and call them on it.

If you want a sense of better protection like Baaad65 was seeking I have a recommendation that most don't know of. Years back, really not that many, Motorcraft petitioned the SAE lube engineers for a new classification.........75w-110. Why would they want to do that? They were seeking better protection and also seeking less friction especially in their truck diffy's.........with the fuel thing and cafe standards and such being such a taxable factor. Well, behold the results, the 75w-110 category was granted and it's range of protection back to our pie chart would throw it running from 10.5 to 12 o'clock. So basically you are assured the protection with the 110 is at the upper range of protection of that pie chart in the 75w-90 category. It isn't at 9 o'clock, just barely, it truly is at the upper range near the 12 o'clock. So now we have more protection, less friction, and we are satisfied we have chose a quality product.

To my knowledge Motorcraft may have not even launched the 75w-110 within their arsenal. Many of their trucks with Eco-Boost have the 75w-90 requirement. 

Here is where we get into brands and such and I am not looking to promote my business. I can help those that want to be helped and I will disclose I am an Amsoil Rep and I work the SEMA and PRI and Mint 400 races for them. I have customers across the land and this isn't a debate on oil. It is merely a recommendation on this 75w-110 viscosity. In my past automotive shop (transmission and auto repair) we often used this with differential larger capacity covers where #140 was spec'ed and achieved better fuel mileage and yet the job towing was also still covered. I also have friends in the Trans-Am series road racing that have raced with me and some of them have taken this opportunity to use this lube with great success. 

BTW, there is a certain percentage of friction modifier in the Severe Gear lineup. Usually, we find use the lube and if you have chatter go back and take the extra step of adding the friction modifier. 

Just another angle that maybe you didn't know.

Tom


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## PontiacJim (Dec 29, 2012)

Greek64GTO said:


> The 10 bolt and the 12 bolt both call for 75w-90, let's call it GL-5 as a modern upgrade. As was noted by Baaad65 if you want insurance (presume protection) you can call for a 75w-140 or a 85w-140. That is the same viscosity I use in my diffy on F350 Super Duty diesel tow rig.
> 
> Now imagine using a 50 weight oil in your engine and pushing the oil pump to increased load and resistance. The same thing in your 10 or 12 bolt or 9 inch. I truly don't think you need to be using a #140 weight gear fluid.
> 
> ...


Greek64GTO: "Now imagine using a 50 weight oil in your engine and pushing the oil pump to increased load and resistance. The same thing in your 10 or 12 bolt or 9 inch."

PJ: Rear ends do not have oil pumps, it is splash lubrication, not the same thing.

"In open gear designs, adhesion is required. Gear oils are one situation where using full synthetic oils isn’t always desirable. Base mineral oils often perform better than synthetics at lower temperatures, having a greater film strength by virtue of a higher pressure-viscosity coefficient. Higher viscosity provides a thicker film, higher wear resistance, and less deformation of the gears as time goes on. While most mineral oils have little to no interaction with ferrous metals, there are a number of parts in any given gearbox/rear axle which may interact with synthetic oils, these include soft metals like brass, rubber gaskets, and seals."

Not rocket science. Our old cars use mineral gear oil, not synthetic. Any auto parts store will have a 75W-90 gear oil or 80W-90. IF you know that you have a "Safe-T-Track", often simply called a "posi rear end," then you will want to add the posi-traction additive as there are clutches inside that require this or your can get rear end "chatter."

Leave synthetic oils to newer cars made/requiring synthetic oils unless you car/engine/trans/rea end has been built for the use of synthetic oils.


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## Hoofer814 (Apr 11, 2020)

Wow, thanks to everyone for their input!


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## Greek64GTO (Dec 13, 2015)

PontiacJim said:


> PJ: Rear ends do not have oil pumps, it is splash lubrication, not the same thing.
> 
> Greek64GTO: I wasn't implying that the diffy had a pump. I merely was giving an example of more resistance of the fluid with a higher viscosity as it is forced into the pinion ring gear combination.
> 
> ...


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## Greek64GTO (Dec 13, 2015)

Apologies to the OP. My intent was to offer a different view on viscosity and not debate synthetics vs mineral base lubes.


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## PontiacJim (Dec 29, 2012)

Greek64GTO: The car was not built for the fluids. The fluids were developed for the problems that occur within the individual application that has occured.
Greek64GTO: Some older cars use mineral gear oil, it is the choice of the owner. When our cars evolved synthetics were just being formulated in the labs. Thus the fallacy of you can not or should not use them. 


PJ: True to a point, but some components such as rubber seals and various gaskets were designed to be compatible with the fluids used, thus different seal & gasket materials were designed for the synthetic formulas/additives which can deteriorate the original/"old" type seals and gaskets. Having the correct chemicals, or incorrect in some instances, are needed for older vehicles as per design. I am not a chemist, but we all know what ZDDP is needed for. Leave it out, or lower it too much and it can $spell problems. Build an engine to overcome synthetics, ie a roller engine, different bearing clearances, use viton seals and recommended gaskets, then synthetics may be of use - I don't know, I use conventional oils in my "old" conventional cars just like the factory did.

Greek64GTO: Great points as always PJ, a true difference of opinion and it will go down in history that some will remain in 1964 and beyond. 

PJ: You must be a 1980's baby. Funny how somethings of "old" worked so well and the "new and improved" is not so new and improved, case in point, my 2015 Hyundai 1.6 GDI aluminum engine that is now going through oil like a baby on a mother's boob - aluminum cylinder walls are scored up; carbon builds up on valves and can drop into the cylinders and act like sandpaper on aluminum, duh (All GM engines use cast-iron cylinder liners, either pressed or cast into the block. That ensures that even with aluminum blocks, the pistons and rings get the benefit of iron’s hardness. “They are definitely subject to premature failure, The most serious problems with engines that use aluminum have come from two sources: designs that match aluminum heads and cast-iron blocks, and *those that do not use cast-iron cylinder liners*.). Figured the industry learned from the aluminum Vega 4-cyl engines in the 1970's - remember those?

Bottom line is that it is not about the differences between mineral oils and synthetics (full or semi),* the difference is in their application based on *the manufacturer, the year built, the manufacturer's recommendations, and during any rebuild, upgrade, or replacement parts - it may be up to the manufacturer of the product, the product design, or the rebuilder who will recommend, or require, a different oil/fluid that deviates from what the original OEM oil/fluid is per service recommendations. I would not put 75W-90 mineral gear oil in my TKO-600 5-speed.

Common sense has to be used once in a while and seems those of old have it and use it more often than the current generation's thinking.


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## Greek64GTO (Dec 13, 2015)

PJ, you are most flatering, thinking me to be a 1980's baby! My grandmother taught me to think young and I am glad that I have projected that!

Not only do I possess common sense, but along with it you might say a bit of wisdom has been stirred in!


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## Hoofer814 (Apr 11, 2020)

Ok! One more question: when I took the rear diff cover off the previous owner used a liquid gasket product. There was no gasket on it so I just cleaned the residue off of the cover. Cleaned the lip and the lip on the gear housing. I put Hi Tac on the cover lip, put a Feldpro gasket on, covered the gasket with Hi Tac and installed it. It dawned on me then that maybe I should have stuck with the liquid gasket. Thoughts? I haven’t added diff fluid yet.


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## Jim K (Nov 17, 2020)

Using some type sealer without the gasket is just an expedient way of putting it back together if you don't have a gasket handy. Probably not the most ideal but a lot of folks do it since there is no "pressure" to speak of.. The factory used a gasket here so I've always used a gasket. Metal surfaces can have defects/flaws in them that a gasket will help seal. I think you did the correct thing.


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## Hoofer814 (Apr 11, 2020)

Jim K said:


> Using some type sealer without the gasket is just an expedient way of putting it back together if you don't have a gasket handy. Probably not the most ideal but a lot of folks do it since there is no "pressure" to speak of.. The factory used a gasket here so I've always used a gasket. Metal surfaces can have defects/flaws in them that a gasket will help seal. I think you did the correct thing.


Thanks Jim. I checked with a friend and he said it will be fine the way i did it. I used non-synthetic Lucas Oil 80W90 fluid, following PJs lead. As you can see, it definitely needed it! I appreciate all of the advice!


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## PontiacJim (Dec 29, 2012)

Hoofer814 said:


> Thanks Jim. I checked with a friend and he said it will be fine the way i did it. I used non-synthetic Lucas Oil 80W90 fluid, following PJs lead. As you can see, it definitely needed it! I appreciate all of the advice!
> View attachment 156377


One thing often overlooked is the axle vent tube. These can clog up over time. If the function of the vent tube no longer works, the rear end can build up internal pressures and that pressure has to go somewhere - out a seal or gasket, which can cause a leak. Check to see if your rear axle has this, believe all do, but look for it. You can get a new/repop replacement if needed. Sometimes just cleaning the original one will work, but they are plastic and can break and I have found them where the top "hat" was missing/broken.


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## Hoofer814 (Apr 11, 2020)

PontiacJim said:


> One thing often overlooked is the axle vent tube. These can clog up over time. If the function of the vent tube no longer works, the rear end can build up internal pressures and that pressure has to go somewhere - out a seal or gasket, which can cause a leak. Check to see if your rear axle has this, believe all do, but look for it. You can get a new/repop replacement if needed. Sometimes just cleaning the original one will work, but they are plastic and can break and I have found them where the top "hat" was missing/broken.


Thanks PJ. I’ll look for the vent tube. Will excess fluid leak out of there after I fill the diff? I came out this morning after completing the work last night and saw a small pool of fluid underneath. I moved the cardboard over to a clean section and went back inside, checked it an hour later. Only a few drips. I thought maybe the excess came out of the vent hole you refer to. I’m going to check it again when I get home from work this afternoon. Please advise!


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## PontiacJim (Dec 29, 2012)

Hoofer814 said:


> Thanks PJ. I’ll look for the vent tube. Will excess fluid leak out of there after I fill the diff? I came out this morning after completing the work last night and saw a small pool of fluid underneath. I moved the cardboard over to a clean section and went back inside, checked it an hour later. Only a few drips. I thought maybe the excess came out of the vent hole you refer to. I’m going to check it again when I get home from work this afternoon. Please advise!



Nope, the vent is at the top. Nothing critical to lose sleep over, just check it and make sure it isn't all plugged up with crud,or simply replace it. When you fill the rear end, make sure the car is level. Only fill it to the bottom of the fill plug on the side of the center section, when the gear oil begins to run back out on you, its full. Use a drip pan under the fill hole. Let the excess drip out (which may be where your "leak" is coming from - excess gear oil that dripped out and is following/clinging to the bottom of the differential) a little if you got carried away and used a high flowing fire hose to fill the gear oil and added too much. If the car is not level, you may overfill/underfill it.

You should not have any fluid leaking out the cover as that is the only place to leak with what you did. Other areas would be the pinion seal at the front/u-joint area and an axle seal. You want to make sure all your cover bolts are evenly torqued down. If one was tighter than others, or others too loose, you could cause an unevenness in the cover and not get a good flat/even seal all around - Service Manual says 25 Ft Lbs of torque for the rear cover bolts.

Just wipe everything clean. I use Brake Clean to get all the crud/fresh gear oil off around the cover and underneath where excess gear oil could be trickling down from. Then place you cardboard under it.


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