98 F-150 9.75" Rear Differential Fluid Questions

[jscott]

98 F-150 w/ 5.4L & 9.85 Rear Axle - 3.55LS Gears

Currently has about 88k on the factory synthetic fluid. The manual says it is a "lifetime" fluid, as long as the axle is not submerged. I plan to change the fluid in the near future, although I am not having any issues with the rear end. The book calls for 75w140 synthetic which I can purchase at the Ford dealer. I have heard that I can use 75w90. I know I will need to add 4.5oz of friction modifier.

Who makes the Ford 75w140 synthetic? Should I refill with this fluid, or go with Mobil 1 75w90?

 

[heyjay]

I'm not sure I can recommend the commercial Mobil MobilLube SHC 75W-90 for a limited slip application. The additive package is incompatible with the clutches inside a LS unit.

I run SHC 75W-90 in my Eaton Dana HP-40 tandems with excellent results, and I maintain the extended 750,000 mile warranty with 500,000 mile drain. This is a FAR different application than a limited slip clutch pack.

If you're interested in keeping the truck a long time, it's FAR cheaper to regularly service the fluids. There is NO SUCH THING as a "lifetime" or "lubed for life" system.

That said, Mobil does make their Mobil 1 Gear Oil in a 75W-140. My local Mobil commercial distributor sells the regular Mobil 1 Gear Oil 75W-90 and the M1 Gear Oil 75W-140 both for $5.25 a quart.

If Ford recommends a 75W-140, then you should probably keep using a 75W-140.

Some LS units seem to benefit from extra LS additive. I've had good luck with CRC Trans-X Posi Trak additive, in a 7 fl oz tube at my local NAPA for $7.50. The CRC additive claims to meet Ford spec.

Be prepared to find some sludge or other nasties inside when you wrestle the sheet metal cover off. Take your time to carefully clean that crap out. Do NOT use solvents of any kind or you will degrade the new gear oil - it will foam and you'll experience rapid gear wear.

Jerry

 

[MolaKule]

quote:

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I'm not sure I can recommend the commercial Mobil MobilLube SHC 75W-90 for a limited slip application. The additive package is incompatible with the clutches inside a LS unit.

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Which I why stated that he should check with George at Avlube. If anything, the EP additive package is more than hefty enough to reduce wear.

My experience has been that the FM package has to be adjusted for each differential with LS additive anyway.

 

[MolaKule]

The Ford oil is good for 100,000 miles, but only after an initial change at 5,000 miles.

You can use the synthetic 75W90's, but here is a list of recommended oils:

Amsoil Series 2000 75W90
Redline 75W90. This has LS FM pack aready in it.
Mobil 75W90,

all available from site sponsers.

I think I would go with the SHC series of oils from Mobil. Contact George at AVLUBES, one of our sponsors, for specifics.

 

[heyjay]

I was concerned as I tried using another Eaton-Dana E500 certified gear oil in my 2000 GMC Sierra with the G80 GovLok.

With the Exxon SGO 75W-90, the GovLok stopped working altogether. It took several flushes with regular 80W-90 and many tubes of CRC additive to bring it back to life.

The Mobil 1 Gear Oil 75W-140 already claims to have the additives. They caution for certain LS applications you may still experience chatter, so extra FM will be necessary:

http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil1_75W-140_SyntheticGearLubricant.asp

For a mechanical locking differential, and for an open carrier, the commercial E500 oils offer a superior basestock and additive package. Since they don't have the FM for clutch plates, expect slightly better fuel economy, much better high temp, and better low temp operation.

Jerry

 

[crashz]

Changed mine at 65K in my F150. What you'll find is a lot of crud. Metal filings and water mostly. The vent is poorly located in one of the bed supports and is prone to suck water in.

 

[heyjay]

quote:

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Originally posted by crashz:
Changed mine at 65K in my F150. What you'll find is a lot of crud. Metal filings and water mostly. The vent is poorly located in one of the bed supports and is prone to suck water in.

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Oh NO water is the worst!

On my 2000 GMC Sierra, the axle vent tube is routed up to the gas fill door. Still, you should regularly check the axle vent tube for cracks. I like to check every 2 years for plugging too.

Did you follow up with another short change interval? If you found water, there will be residual water in the axle shaft housings. The only way to get the water out is change to fresh gear oil.

Jerry

 

[crashz]

I only changed it once but will do it again this spring. I changed it with M1 75w-90 mostly for mileage purposes, but will go back to 75w-140 (the stock Ford fill) for towing. I'll use the M1 as 10K flush.

I know where the water came from. I'm very meticulous when washing my truck, and would spry the underside of the bed down at least every week. Each time I would essentially fill the bed support rails, flushing all the road salt out. In the process, the water would get forced down the vent. I'm much more careful now, but the long term solution is to move the vent.

 

[sbc350gearhead]

I have 2 aftermarket eaton posi units, and never had to add any FM additive to either RP-maxgear, or lakewood BFL.

 

[heyjay]

quote:

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Originally posted by sbc350gearhead:
I have 2 aftermarket eaton posi units, and never had to add any FM additive to either RP-maxgear, or lakewood BFL.

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If the gear oil already claims "suitable for limited slip service" then you shouldn't have to add more LS additive.

Some LS units, especially ones that are getting worn, may require additional LS additives to prevent chatter or slip.

Are your Eaton Posi units the older style clutch packs or the newer Carbon Fiber clutch packs. It does seem the newer carbon fiber clutches require much less additive.

Jerry