Experts Chime In: Petroleum or Synthetic Gear oil for my LSD

[Billy00NC]

I recently swapped out my rear end axle (LSD) with a low mileage used one. I cleaned out the pumpkin and filled it with synthetic gear oil that included Friction Modifier. After 5k miles I started getting the dreaded chatter/rumble on turns during take off. I emailed a Differential retailer about adding more Friction Modifier and he replied "...Ford recommends synthetic oil and we DO NOT. Synthetic oil has the tendency to block the additive from getting to the clutches. We recommend 80/90w gear oil and the Ford additive."

Is there anyone else out there that has heard or read any similar advice?

I'm going to add some friction modifier soon to my current mix, but if I get some confirmation I may swap out my synthetic for "dino" oil if it can give my clutches longer life.

 

[SoNic67]

How many LS differentials Ford put on roads, and have no issues with the synthetic factory fill (including mine)?
Ford manual and label on the differential says for the rear with LS "SAE 75W-140 High Performance Synthetic Rear Axle Lubricant" and... that's what it should be used - per foot note 6:

Vehicles equipped with 4.0L SOHC V6 engines and 3.73:1 or 4.10:1 rear axle ratios or with a 5.0L V8 engine require synthetic rear axle lubricant.
Rear axles containing synthetic lubricant are lubricated for life. These lubricants are not to be checked or changed unless a leak is suspected or service is required. The axle lubricant should be changed any time the axle has been submerged in water.
Add 118 ml (4 oz.) of Additive Friction Modifier C8AZ-19B546-A or equivalent meeting Ford specification EST-M2C118–A for complete refill of Traction-Lok axles.

 

[Flash]

Any oil, synthetic or otherwise, has the "tendency" to block other additives only if it has the sort of additive in it that will do that.

Any oil, synthetic or otherwise, is base oil plus additives.

Use the correct synthetic oil plus the correct additive and you will have life long lubrication and no lubrication caused problems.

 

[swshawaii]

Seems only aftermarket axle suppliers recommend conventional gear lubes. Every OEM I've seen specifies synthetic on newer vehicles.

http://www.bobistheoilguy.com/forum...61/Re:_Why_A_Conventional_Over_a_#Post2877261

 

[SoNic67]

Synthetic oils have less additives than "normal" oils so they are less likely to react negatively to LS additives and they also because of that they have less compounds to break-up during life of the oil.

 

[rb142]

Frankly, put in whichever you want, along with the right friction modifier. The aftermarket additives don't all work the same as the Ford stuff. If you have the chatter, you need the friction modifier whether the fluid is dino or synthetic.

Ford originally spec'ed 80/90 on these vehicles, and moved to 75/140 later to better handle heavy loads and towing. The synthetic recommendation likely had more to do with the "don't need to change this for life" recommendation than anything else ... and that most 75-140 type oils happen to be synthetic.

It may be that the additives in some of the aftermarket oils don't interact well with the Ford friction modifier, but I haven't heard anyone complain about it. If you just add some of the Ford friction modifier in there it will probably be fine.

 

[Billy00NC]

I am adding Motorcraft XL3 (part #C8AZ-19B546-A ) as soon I receive it to my current synthetic 75w140 differential content. Hopefully this will cure my rumble.

I feel the need to repeat that this is my SECOND rear axle assembly. The first rear end whined loudly when I bought the Explorer at 140k miles. I had bearings replaced and synthetic oil and friction modifier filled. The whining continued until the differential fell apart at 190k. The replaced axle had 74k when it was installed with fresh synthetic that included friction modifier. I hope my experience shows my efforts are only for the protection of my differential's internal parts and also the protection of the contents of my wallet.

________________
"Once competent participants were shown the work of others, they were able to revise their performance and gauge better their percentile ranking" (also Kruger and Dunning)

 

[SoNic67]

My axle is original and has 140k miles - no issues whatsoever.
I wonder if anything else is wrong in your rear end to lead to this?

 

[BustedKnuckles24]

At 225,000 miles, I changed my rear axle lube. I used Valvoline Synthetic 75W140 and Trans-X Limited Slip additive. My truck has 257,000 miles on it now, and rear axle is as quiet as a church mouse.

 

[GRNMACHINE]

I used Royal Purple 75w140 when I replaced my rear axle cover with an aftermarket one with drain plug. Added the Ford Friction Modifier even though most said it was not needed. That was a few years back and no chatter. Oil was really pricey though.

 

[chefduane]

I just flush/filled my 193k mile 3.73 LSD with Valvoline Full Synthetic 75w-140 and Sta-Lube Equa-Torque (Ford spec) friction modifier. I also changed the gasket and put on a shiny new cover. Dunno' if it needed it but it sure looks purdy!

 

[masospaghetti]

Dino or synthetic is probably fine, although personally I would definitely stick to the correct weight (75w-140) and not 80w-90.

 

[Billy00NC]

Thanks for all the input. For now I'm sticking with 75/140 synthetic and Motorcraft XL3 FM.

I believe I may have found the source of my rear end "rumble". When a mechanic and myself replaced the rear-end we filled it with gear oil and additive, but when I recently checked it, the oil level was low. It took the four ounces of XL3 and an entire quart of gear oil to fill it up. My mechanic believes the axle chambers weren't filled until I drove it awhile.

After filling I still have the rumble, but believe it's possible it might go away after putting some real time miles on it.