Differential Gear Oil Weight

[Kodachrome Wolf]

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.

[Lincolnmania]

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.

[turbo2256b]

My ford factory repair manual recommends heavier grade in performance applications.

[IPreferDIY]

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.

[Kodachrome Wolf]

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.

[nathand]

I use 75W140 also and have no complaints whatsoever.

[GM_Guy]

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.

[DerekTheGreat]

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.

[gadget73]

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.

[IPreferDIY]

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.

[BigMerc96]

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.

[Tiggie]

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.

[DerekTheGreat]

Must be made from refined whale blubber, walrus & elephant tusks.

[Brown_Muscle]

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?

[Kodachrome Wolf]

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.