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Thread: Differential Gear Oil Weight

  1. #1
    Road Warrior Kodachrome Wolf's Avatar
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    Default Differential Gear Oil Weight

    I'm planning on changing the rear diff fluid in the wagon again just to ensure it's staying clean back there after it got new bearings installed.

    I know the typical oil weight is 80W-90. However, I know some later (like '07+) police package Crown Victorias were listed to use 75W-140. I'm currently using the 75W-140 in my '97 over the regular 80W-90. Haven't had any problems thus far in that car.

    However, I'm wondering if the 75W-140 would be okay in an older axle. I'm just trying to extend the service life of components where possible.

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  2. #2
    Domestic Tourist Lincolnmania's Avatar
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    i have been running mobil 1 75w90 in everything for years.....no issues.

    Should be able to run 75w140.....it is the same axle either way.

    1986 lincoln towncar signature series. 5.0 HO with thumper performance ported e7 heads, 1.7 roller rockers, warm air intake, 65mm throttle body, 1/2" intake spacer, ported intakes, 3.73 rear with trac lock, 98-02 front brake conversion, 92-97 rear disc conversion, 1" rear swaybar, 1 3/16" front swaybar, 16" wheels and tires, loud ass stereo system, badass cb, best time to date 15.94 at 87 mph. lots of mods in the works 221.8 rwhp 278 rwt
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  3. #3
    I post a lot... turbo2256b's Avatar
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    My ford factory repair manual recommends heavier grade in performance applications.
    Scars are tatoos of the fearless

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    Goal was 'giving back', done by 2020.
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    For my 2000 MGM, the owner's manual says 80W-90, but the maintenance schedule implies that 75W-140 is so much better for special operating conditions:

    Normal Vehicle Axle Maintenance

    Rear axles containing synthetic lubricant and light duty trucks equipped with
    Ford-design axles are lubricated for life. These lubricants are not to be checked
    or changed unless a leak is suspected, service is required or the axle
    assembly has been submerged in water. The axle lubricant should be changed
    anytime the axle has been submerged in water.
    Non-synthetic rear axle lubricants should be replaced every 3,000 miles or
    3 months, whichever occurs first, during extended trailer tow operation above
    21 Celsius (70 degrees Fahrenheit) ambient and wide open throttle for extended
    periods above 45 mph. The 3,000 mile lube change interval may be waived
    if the axle was filled with 75W140 synthetic gear lubricant meeting Ford
    specification WSL-M2C192-A, part number F1TZ-19580-B or equivalent. Add four
    ounces of additive friction modifier C8AZ-19B546-A (EST-M2C118-A) or
    equivalent for complete refill of Traction-Lok rear axles. The rear axle lubricant
    should be changed anytime the axle has been submerged in water.

    Police and Taxi Vehicle Axle Maintenance

    Replace rear axle lubricant every 160,000 km (100,000 miles). Rear axle
    lubricant change may be waived if the axle was filled with 75W140 synthetic
    gear lubricant meeting Ford specification WSL-M2C192-A, part number FITZ-19580-B
    or equivalent. Add four ounces of additive friction modifier C8AZ-19B546-A
    (EST-M2C118-A) or equivalent for complete refill of Traction-Lok rear axles. The
    rear axle lubricant should be changed anytime the axle has been
    submerged in water.

    2000 Grand Marquis LS HPP, a hand-me-down in 2008 with 128,000 km; 175,000 km as of July 2014
    mods: air filter box 'tuba', headlight relay harness, J-mod (around 186,350 km), 70mm throttle body, KYB Gas-A-Just shocks, aluminum driveshaft, ARA3 PCM

  5. #5
    Road Warrior Kodachrome Wolf's Avatar
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    Sounds like I'll run with the 75W-140. Figure I would ask around first before doing it on that axle.

    The only down side will be the incredibly negligible impact it'll have on fuel economy.

    My Cars:
    -1964 Comet 202 (116K Miles) - Awaiting Resurrection
    -1987 Grand Marquis Colony Park LS (325K Miles) - April 2017 + September 2019 POTM Winner
    -1997 Grand Marquis LS (230K Miles) - The Daily Workhorse & March 2015 + January 2019 POTM Winner

  6. #6

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    I use 75W140 also and have no complaints whatsoever.

  7. #7
    Stow It! GM_Guy's Avatar
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    Get whatever synthetic you find. 80w90, 75w90,75w140, doesn't matter much. Your nose will thank you for the synthetic whenever you need to remove that cover again in the future.

    Alex.

  8. #8
    GMN Regular DerekTheGreat's Avatar
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    With that said, I always thought any of that gear oil was good for ~100k miles. I have no idea, save for going back & looking at the receipts, what was used in our cars. Well, synthetic or not. Didn't realize the non-synthetic stuff was only good for 3k miles. Bah.
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  9. #9
    I'm an air-conditioned gypsy gadget73's Avatar
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    Thats under very specific conditions though.

    "Non-synthetic rear axle lubricants should be replaced every 3,000 miles or
    3 months, whichever occurs first, during extended trailer tow operation above
    21 Celsius (70 degrees Fahrenheit) ambient and wide open throttle for extended
    periods above 45 mph. "

    Unless you tow all summer long and mash the pedal constantly its good for more than 3K miles. The synthetic will take that abuse though.

    I use synthetic because its honestly not much more expensive. I don't know that it does miraculous things but for an extra couple bucks it doesn't feel like a waste.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

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  10. #10
    Goal was 'giving back', done by 2020.
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    The maintenance schedule actually implies that the non-synthetic is good for 100,000 miles in police and taxi use:

    "Police and Taxi Vehicle Axle Maintenance

    Replace rear axle lubricant every 160,000 km (100,000 miles). Rear axle
    lubricant change may be waived if the axle was filled with 75W140 synthetic ..."

    The maintenance schedule also implies that even non-synthetic doesn't need to be changed for regular (i.e. non-police or taxi) use, but I don't see why anyone would want to leave it for more than 100,000 miles. Mine was a nasty gray color well after that point, and I feel a lot better about having fresh stuff in there.

    2000 Grand Marquis LS HPP, a hand-me-down in 2008 with 128,000 km; 175,000 km as of July 2014
    mods: air filter box 'tuba', headlight relay harness, J-mod (around 186,350 km), 70mm throttle body, KYB Gas-A-Just shocks, aluminum driveshaft, ARA3 PCM

  11. #11
    Member BigMerc96's Avatar
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    I always run 80w90 or 75w90 synthetic. Town Car's diff had no oil in it when I got it, no idea how long it had been like that, no signs of leaking and it was quiet until it ate an axle bearing 10k miles into my ownership. I put conventional 80w90 in it after replacing the bearings in it and all was well with the world aside from a slight whine at very specific speeds, notably 46-52mph and 75+mph. Fast forward a couple years and 50k-ish miles and I ran over something and mangled the diff cover so it was replaced. When I did that I bought Valvoline 75w90 synthetic for the refill and much to my surprise the whine decreased noticeably, its still there, just quieter. So, ever since then I've run the 75w90 synthetic in any 8.8s I've serviced since. Its not much more expensive to run the synthetic stuff if you catch it on sale and its a bit of piece of mind IMO, if it made the whine of a neglected/abused diff quieter then there has to be something to it.
    -Steve

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  12. #12
    Wagon Addicted Tiggie's Avatar
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    My dad believes his 51 Ford has the original gear oil in it. He's had the car since 1972ish and the prior owner from 53-him wasn't much on maintenance. I doubt the original owner changed it in the first two years. It's his claim to gear oil fame right now. Pennsylvania crude maybe? He drives it 50+ miles a week so it's not a garage queen either. I will change it if it becomes mine.
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    GMN Regular DerekTheGreat's Avatar
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    Must be made from refined whale blubber, walrus & elephant tusks.
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  14. #14
    P31 Pursuit Car Brown_Muscle's Avatar
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    Thread resurrection

    You guys using the 75w-140 synthetic, did you add friction modifier if you have limited slip? I know most say it comes with it but i've been reading conflicting things... I'm going to change mine and have a bottle of ford friction modifier on the shelf... would it decrease the clutches effectiveness if i add it to fluid that already says it has it in it?
    -Phil



    +1982 Ford LTD-S Police Car. Built 351w, Trickflow 11R 190 Heads, Edelbrock 1906 Carb+ RPM Intake, WR-AOD, Full exhaust headers to tails. 3.27 Trac-Lok Rear. Aluminum Police Driveshaft. Speedway Springs+Bilstein Shocks, Intermediate Brakes, HPP Steering Box.

    +2003 Acura CL Type S 6-speed

  15. #15
    Road Warrior Kodachrome Wolf's Avatar
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    Both of my cars are open diffs, so whether or not the stuff has the friction modifier in it was of no difference.

    On that matter, however, I’d start with the fluid first if it already has friction modifier in it. As I’ve heard, if it chatters any after adding the fluid with FM already there, add like an ounce of FM and test it again. Usually the fluids with it already added are pretty solid and typically do not need more added.

    My Cars:
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    -1987 Grand Marquis Colony Park LS (325K Miles) - April 2017 + September 2019 POTM Winner
    -1997 Grand Marquis LS (230K Miles) - The Daily Workhorse & March 2015 + January 2019 POTM Winner

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    Quote Originally Posted by Brown_Muscle View Post
    Thread resurrection

    You guys using the 75w-140 synthetic, did you add friction modifier if you have limited slip? I know most say it comes with it but i've been reading conflicting things... I'm going to change mine and have a bottle of ford friction modifier on the shelf... would it decrease the clutches effectiveness if i add it to fluid that already says it has it in it?
    I was told that you don't need to add the friction modifier to synthetic Phil. Used Redline in both of my Marauders when I upgraded to 31 spline refurbed trac locs & the clutches were really noisy . Added friction modifier to both clutch noise eventually went away & saw no loss of clutch effectiveness.

  17. #17
    I'm an air-conditioned gypsy gadget73's Avatar
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    I don't think there is friction modified in the Towncar and it chatters a bit when making tight turns in parking lots. Won't hurt anything to put it in there.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

  18. #18
    Beater gonna beat sly's Avatar
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    Mine chattered without friction modifier for 45K miles. Serviced and the shop used friction modifier and now it's quiet but just chirps the tires around corners if I big toe the gas. Most likely that's due to the 8-clutch setup in mine.

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  19. #19
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    Quote Originally Posted by Kodachrome Wolf View Post
    I'm planning on changing the rear diff fluid in the wagon again just to ensure it's staying clean back there after it got new bearings installed.

    I know the typical oil weight is 80W-90. However, I know some later (like '07+) police package Crown Victorias were listed to use 75W-140. I'm currently using the 75W-140 in my '97 over the regular 80W-90. Haven't had any problems thus far in that car.

    However, I'm wondering if the 75W-140 would be okay in an older axle. I'm just trying to extend the service life of components where possible.
    I'd stick with the 80W-90 if it were mine. Why go to the thicker goo if you don't have to? Are you towing with the wag?

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