After searching both here, and MC.net I’m still scratching my head as to WHAT fluid to use in my differential; there appears to be no authoritative answer, just a lot of speculative opinions (although I do not discount those as some of my cohorts on these sites are experientially rich; however I haven’t found much conclusive agreement.) That being said, I asked “Curry” (currieenterprises.net) referencing a 1970 open 9" Ford rear, presuming they of all entities would have a definitive answer. Result: “all the Ford 9-inches use the same oil” (in reference to vintage of rear) - “3 QUARTS OF 85/140 GL 6 OR 5, NON-SYNTHETIC”.
I always use quality synthetic over Dyno oil. I’m surprised to see 85/140. 75/90 was the classic viscosity for a long time. If it’s an open differential, there’s nothing special there. 75/90 or 80/90 - no one will ever see the difference in those. A 140 weight will simply cause you (in a small way) to burn more gas.
I don’t know why the 85/140 either; but being in FL it probably isn’t a horrible idea. My understanding is that synthetic oil’s lubricity is inadequate for the differential. I personally don’t know. I also thought (by posted advice) that GL5 was contraindicated as well
80/90 non synthetic.
I use this Brad Penn 80W90 in my 3.25 open. Shop manual calls for 80W90.
I just filled the rear for my '67 GT this week. It’s a 9 inch 3.00 gear with an aftermarket Eaton Detroit Trutrac unit. I used 2.5 qt of Lucas 85/140 as recommended by my builder.
Sean
Engineer speaking here - bunk to whomever is telling you what they are saying. Regardless of the base (dyno vs. diester vs. other) - 85 weight is 85 weight. The viscosity is NOT relative to the base of the gear oil. That is just like the myth that premium gas helps cars that run on 87 octane. No benefit. Your choice.
The GL5 spec came out much later. All the requirements that Ford needed when they designed that 1970 rear end were satisfied by the spec they dictated back then. I’ve not researched GL-5, but I highly doubt there is anything in it you need. If you can get an oil that satisfies both specs and that makes you happy - go for it.
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Appreciate that. Either for further muddling, or for clarity - unsure which, here with the substantiation from Currie:
We get asked this question almost every day, and the answer is straight up, you should only run 85W-140, API GL-6 rated conventional (non-synthetic) gear oil in a Currie axle assembly. You may hear about how synthetic gear oil provides reduced friction and wear; however, it may not be optimal for your setup – especially if you have a Ford 9-inch rearend. Hear us out.
The 9-Inch Gear Is Unique
The design of the 9-inch features a high-offset distance that places the center-line of the pinion gear 2.25-inches below the center-line of the ring gear, which is substantial when compared to the 1.125 to 1.5-inch offsets of other gear sets (see image below). The high-offset distance increases the contact patch between the ring and pinion teeth by approximately 30-percent and is a crucial reason for the legendary strength of the 9-inch gears. However, strength comes at a price, as this high-offset distance creates increased sliding friction and heat across the gear tooth face. Because of this, not using high-shear oil such as Currie’s 85/140w can lead to scoring and certain gear death, especially during extreme scenarios such as off-roading and racing. The increased pressure capabilities and wear resistance found in Currie’s GL-6 rated gear oils are designed with all this in mind to ensure your rearend’s gears and bearings are adequately protected under the most severe conditions.
Pinion offset distance compared
API GL-5, Not Designed For A 9-Inch
Virtually all synthetic gear oils on the market today are only engineered to meet the API GL-5 specification, which Currie has found to be inadequate for the design of the 9-inch. The last 9-inch rearend rolled off the Ford assembly line in 1986, and it wasn’t until 3-years later 1989 that the first synthetic gear oil, Mobile1 (GL-5 rated), hit the market. Just 9-years after the end of 9-inch production, in 1995, both the GL-6 standard and the test equipment were inactivated, making it unlikely that a synthetic oil manufacturer would go through the cost and effort to develop an oil to meet the obsolete GL-6 standard.
Conventional Oil Is Proven
Because many of today’s commercially available synthetics are not engineered with the 9-inch’s requirements in mind, Currie advises using only conventional 85W-140 non-synthetic GL-6 rated gear oil in all front and rearend applications. The viscous (thick) base stock of the 85W-140 oil (check out the videio)provides a cushion that helps protect the gears from shock loads and clings more effectively, pulling heat away from the tooth face and preventing localized heat build-up. The higher level of extreme pressure additives mandated by the GL-6 standard protects the gears from scoring and premature wear.
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I’ve never bought anything from Currie. All my Ford 9" rears use this:
If the differential has a Traction Lock I use the Ford differential anti-friction additive.
Ford’s limited slips for the longest time have needed supplemental ‘friction modifiers’. His is an open diff.
Retro1 - if you’re happy with what the other folks told you, then have at it.
With an open diff, any gear oil that meets the original spec will be fine. Yes, synthetic is better, but not needed. The reality is that you’ll likely not put enough miles on it in a lifetime to matter; but the synthetics generally don’t brake down (100% synthetic at least).
Royce/Inmanlanier: Thank you both for the input; I’m just trying to make sure I don’t lunch anything. I like going with OEM Ford, and it looks like National Parts Depot carries it, I may just check the price at the local Ford Dealer for comparison. Thanks again.
If I had my druthers all differentials would be Detroit Lockers. Then regular oil is fine. Unfortunately if a car came with a Traction Lock I am forced to keep it - usually.
I am fortunate in many regards, but one in particular relevant to this statement:
When I bought my present house (likely my last) I had been looking for acreage (5 - 10) and with it would have come sizeable outbuildings and an unchecked desire to accumulate (cars would be a priority); instead I bought less than an acre, on waterfront (fresh). Point is, environmental constraints on waterfront property in Florida precludes me from putting up any more “accessory buildings”, thereby effectively clipping my collecting wings. It should sadden me, but instead I’m thankful because it would just be more for my kids to get rid of when I leave terra firma.
I called the local Ford Dealer parts department and asked about C6AZ-19580-B and they had nothing on it. After discussion they told me what they have is good enough, but I’m curious Royce - where do you buy it? Or are you archiving from earlier days?
Looks like this hypoid gear oil ( C6AZ-19580-B ) is available - I copied / pasted this from NPD :
GEAR OIL, SAE 80w-90, Motorcraft Premium Hypoid Gear Rear Axle Lubricant, 1 quart bottle, GL-5 spec non-synthetic, An excellent multi purpose extreme pressure lubricant recommended by Ford Motor Company meeting warranty requirements, It is recommended for conventional and limited slip differentials of passenger car, light trucks and vans that operate under mild to severe service conditions, Manufactured using premium quality hydroprocessed base oils and performance additives to provide thermal and oxidative stability and protection against gear wear, rust and corrosion, This product meets the requirements of WSP-M2C197-A, GL-5, MT-1MIL-PRF-2105E and Mack GO J, Note that some limited slip axles also require the addition of a friction modifier- see F-19546-1, Always add modifier first then fill with gear oil, 8 inch requires about 2 quarts, 9 inch requires about 2.5 quarts, 2 pints = 1 quart,
NPD Part Number : 19580-1
Manufacturer Reference #'s: Motorcraft XY-80W90-QL, XY-80W90-QL, C2AZ-19590-A, C1AZ-19580-E, C1AZ-19580-F, C2AZ-19580-D, C6AZ-19580-B, C6AZ-19580-C, EOAZ-19580-AA, E0AZ-19580-B, E0AZ-19580-C
Also showing up on Motorcrafts website : https://www.motorcraft.com/us/en_us/home/our-products/chemicals-and-lubricants/transaxle-fluid/sae-80w-90-premium-rear-axle-lubricants.html
I have bought it online for years. Like gah says it is easily found everywhere and anywhere. Not sure what your Ford dealer’s problem is - maybe a clueless guy that you talked to? Certainly the dealer rebuilds rear ends every week - what do they use?
Yes, I found that one as well, and NPD is in my neck o’ the woods so no shipping.
ROYCE: They use their present day gear oil, which he had to physically check to tell me it was hypoid; I don’t know the part # however. I was just commenting on the bottle you showed in the post, that looks like “old school”.
It is a label picture that I kept after using the oil inside but the part number is currently available so I do not know what is wrong with your Ford dealer.
Very interesting…
The motorcraft 80w90 is the stuff I use. Motorcraft uses their own numbering system( XY-80W90-QL) rather then the Ford Long number (C6AZ-19580-B). Most dealers should stock it still however its becoming less common for them to sell. I still buy it by the case once or twice a year for work