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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 03-30-2009   #1 (permalink)
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Default What's your favorite rear differential fluid?

The guy at the local auto parts store told me to just throw in some generic 90 weight in, and a bottle of Lucas high mileage diff fluid.

I've had one recommendation on the board for Royal Purple.

What do you think is a good quality rear differential fluid?
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Unread 03-30-2009   #2 (permalink)
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I like royal purple rear end gear oil, been using it for years now. Also mobile 1 syn with ford friction modifier is good too.
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Unread 03-30-2009   #3 (permalink)
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What about Amsoil or Lucas?

Anyone just use generic oils?
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Unread 03-30-2009   #4 (permalink)
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Can't go wrong with Royal Purple. I personally don't run any generic fluids in any of my cars except maybe washer fluid. I'm from the you get what you pay for school of thought. Amsoil, Valvoline synthetic, castrol synthetic, redline etc. are really all pretty good stuff. I run a ratchet type diff so I don't use the friction modifiers so I can't help you there. I do run a little bit of Lucas oil just to keep things coated.
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Unread 03-30-2009   #5 (permalink)
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I use Royal Purple products.I suggest to do the engine,trans,and rear end.
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Unread 03-30-2009   #6 (permalink)
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Unless you are going to run the Baja 500 any 90 weight GL5 will be fine. Gear oil is pretty much a commodity these days; go to WalMart and pick your favorite brand. If it has limited slip you will need to also add a bottle of the magic Ford additive which runs around $5. Ford is the best and easiest place to buy it. Otherwise the limited slip plates will grunt and groan - unless of course they are worn out in which case you no longer have limited slip and you can skip the additive.
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Unread 03-31-2009   #7 (permalink)
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Thanks for the feedback!

What are your thoughts on the Lucas High Mileage Fluid that was recommended to me. I typically shy away from high mileage fluids, but I know Lucas is supposed to be good stuff. Just not sure what they've done to it to make it "high mileage." And if the differential isn't making nay noise, do I really need anything special?
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Unread 03-31-2009   #8 (permalink)
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I have never used any of the Lucas, or much of any 'high mileage' products or additives. In general, I don't see the reason or any potential benefit except to the marketing types who live off the additional sales. Oil is for lubrication. If the gears, rings, whatever, are worn no additive or special elixir can reverse that fact. The hope that there is a 'magic cure' helps sell these products.

Looking at another thread I think you have a limited slip diff. Those need the Ford friction modifier additive or they make noises when you turn corners forcing the clutches to slip. If the clutches still work, i.e., the rear wheels won't easily turn independently of each other, and it doesn't make noises I would leave it alone. If the wheels turn easily, one at a time as mine did, then the friction plates need to be replaced. I don't have a 9" but on the smaller diffs that isn't terribly difficult if you don't mind taking a differential apart. My plates were completely shot since the original owner had limited slip for driving through the snow with chains to his ski cabin at Taos for 14 years. He did it weekly and the limited slip was needed often.
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Unread 03-31-2009   #9 (permalink)
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Did your rebuild just consist of pulling out the old discs and fitting in new ones?

Did you rebuild the entire differential (not sure what all that entails...bearings I would assume)?
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Unread 03-31-2009   #10 (permalink)
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how many miles is on your car? They have rebuild kits for the 8.8 and you can get the carbon clutches that come in the 03-04 Cobra's, even a new unit is like 250 bucks, not bad all all. If you've never worked on the rear end, i would leave it to the professionals. If you plan on a gear swap in the future, i would see about the rebuild or getting the new posi.
For the gear oil, i have never used lucas and probably won't. I stick to what i know that works. but like my first post, i'm a RP fan.
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Unread 03-31-2009   #11 (permalink)
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It has about 170K. It's an oem 9" with oem limited slip.
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Unread 04-01-2009   #12 (permalink)
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Your diff is not the same as mine so I can't say for sure exactly what you have for clutches. Because of the 2.41:1 gears my ring gear is only 7 3/4" although it is generically an 8". It has a set of alternating steel plates and friction plates clamped together under a giant size Belleville washer on the back side of the ring gear. They are not all that difficult to change. If you are changing gearsets or bearings then you have to have, or acquire the knowledge and tools to set the ring and pinion spacing. If all you are doing is putting new clutches in the limited slip its not that difficult a job. My bearings were fine so didn't change them. The only bearings I have ever replaced on a Ford rear end are the outer axle bearings which take a pounding from the wheels. You will need new seals any time you take anything like this apart but those aren't hard to get. If you can overhaul an engine you can work on a diff.

Some diffs are setup using a one-time-use crush spacer but I think these Fords all use shims. In that case, unless you find something wrong the gears will be going back with the same spacers they already had. How the teeth mate may be an issue for noise depending upon wether you have a ratio which hunts. If you mark and mate the teeth as they were when you took it apart that is not a problem. Even if you are changing a gearset a Prussian Blue tooth check is usually all that you need to do to set up this kind of gears.

Unless your car has led a very easy life I would doubt that the limited slip is still working after 170k miles. Mine was completely shot after 90K but then it did a lot of miles slipping, sliding and spinning in the snow.
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Unread 04-01-2009   #13 (permalink)
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I just had a '69 9" tracloc built for me and discussed this with the builder. He gave me the same advice as I've read for auto trans. - he wouldn't use synthetic in clutches as it is slips more than dino oil, you need friction for clutches to work. I have read elsewhere that unless the auto trans came factory with syn. use dino as the syn. is too slippery, same for tracloc clutches.
I'll be using a quality dino oil in mine but using syn. in my engine after break in.
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Unread 04-15-2009   #14 (permalink)
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Jonk67, what dino oil are you considering?
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Unread 07-14-2012   #15 (permalink)
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Hey guys,

Wanting to change my diff oil but not sure if I have a standard diff or an LSD. Is there some way of checking apart from just relying on the door tag or build sheet?
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