Which gear oil...? - Vintage Mustang Forums
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post #1 of 10 (permalink) Old 09-06-2019, 12:24 PM Thread Starter
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Which gear oil...?

I screwed up and ordered two different brands of gear oil for my 9" rear. I can't take one or the other back because they were ordered in August of 2018. I ordered PennGrade and Lucas 80W-90.


I don't think I can go wrong using either, but would like to know if anyone used either brand. I also have the Ford Fluid XL-3 Friction Modifier Additive to add to the rear as well.


Thank you...
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post #2 of 10 (permalink) Old 09-06-2019, 01:35 PM
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Either should work fine IMO.

1966 Convertible restomod in progress
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post #3 of 10 (permalink) Old 09-06-2019, 01:52 PM Thread Starter
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Either should work fine IMO.
Thank you
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post #4 of 10 (permalink) Old 09-06-2019, 01:58 PM
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Either or. Wouldn't bother me to mix them even, there's very little difference between brands of conventional 80W-90 oil. Because it doesn't cost much more I like to use synthetic these days but it doesn't really matter. The main thing is you have the best friction modifier money can buy.
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post #5 of 10 (permalink) Old 09-06-2019, 06:17 PM
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Assuming they are both "conventional" oil- if they are 80w-90 they are conventional, not synthetic- I would not hesitate to mix them. For all we know they were both made by the same oil company and sold under different private names. Do not mix conventional with synthetic. Synthetic will be 75w-90 or 75w-140.
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post #6 of 10 (permalink) Old 09-06-2019, 08:36 PM
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Either or. Wouldn't bother me to mix them even, there's very little difference between brands of conventional 80W-90 oil. Because it doesn't cost much more I like to use synthetic these days but it doesn't really matter. The main thing is you have the best friction modifier money can buy.
Not to hijack but, is the friction modifier recommended for all rearends or just the 9" ? I just changed the fluid in my little 6 cyl rearend and only used straight 80w-90. Should I have added a friction modifier?

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1966 Blue Hardtop. 200 CID with 3 Speed Trans
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post #7 of 10 (permalink) Old 09-06-2019, 09:05 PM
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Not to hijack but, is the friction modifier recommended for all rearends or just the 9" ? I just changed the fluid in my little 6 cyl rearend and only used straight 80w-90. Should I have added a friction modifier?

Only if you have a posi.
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post #8 of 10 (permalink) Old 09-06-2019, 10:00 PM
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Originally Posted by gduncan View Post
I also have the Ford Fluid XL-3 Friction Modifier Additive to add to the rear as well.
I had an idea that someone east of me bought some friction modifier. I caught a whiff of it while mowing the yard today.

I think Ford formulated the smelliest of stuff on Earth when they made that product.

I have a bottle sitting on a shelf in my workshop. Still waiting on my hazmat suit to arrive from Amazon before I get out my 10' pole with the grabber claw...

Bill

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Color: Y
Trim: 22
Axle: 6
Trans: 6
Build Date: July 18, 1966


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post #9 of 10 (permalink) Old 09-06-2019, 10:05 PM
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Only if you have a posi.

If you have a posi you need to buy whatever Govt. Motors recommends. If you have a Traction-Lok you need the Ford/Motorcraft stuff.
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post #10 of 10 (permalink) Old 09-07-2019, 12:25 AM
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Yep, GM specifies their own additive for "posi's".

You use the additive for any "clutch" style limited slip rear differential. "Locking" style rears don't need it, nor do the most common "open" rears. You might look up lockers, very few people use them in old Fords, but they are out there. Known for making rather interesting clanking noises in corners. Open rears are the ones when when you spin the tires, you actually only spin one of them.

At one time Ford sold THE best friction additive. Seriously. Even the GM and Mopar guys would slink into the Ford dealerships to buy it. That was a long time ago. Today GM and Mopar have their own additives which (much as I hate to say it) are quite interchangeable and as good as Ford's. BUT, the generic branded stuff in the parts stores is worse than useless. Do not buy it. Buy from whichever one of the Big Three has a friendly parts counter nearest to you. I recommend going to Ford just so a stray shiny new Hellcat or Corvette won't tempt you away from the fold though.

Oh, I almost forgot. There exists gear oil which is labeled as having the additive included. I suspect it's one of those deals where one drop added to every 1000 gallons is enough to keep them from getting sued for false advertising. But not enough to do us or our rear ends any good. Avoid those oils.
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Last edited by GypsyR; 09-07-2019 at 12:28 AM.
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