Differential Fluid: Conflicting Information


RangerJoey

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Hi All,
I have a 8.8" 4.10 LS differential and was going to drain & fill the rear differential. My shop manual calls for 5 pints of 80W90 oil and some friction modifier. But I saw this:

https://www.youtube.com/watch?v=-LIm-YZ4DUI

At 16:20, he mentions Ford is going to 75W140. Is that right for this 1999?

And side note, if it calls for 5 pints, is that with the friction modifier for the LS?
 


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Dirtman

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There's no need for friction modifier if you run a synthetic 75w-90 like mobil 1 or Amsoil. There is also no need to run 140, good luck even finding it anymore. Stick with mobil 1 or amsoil 75w available at any parts store, its way better than old fashion 80w90 that need friction modifiers and runs cooler than 140.
 
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Craig0320

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Mobil one already has friction modifiers in it. 80w90 is fine. The friction modifiers keeps the clutches from sticking and reduces vibration and grinding. I put regular 80w-90 in mine and had all those symptoms. Got some mobil one and never had another problem.
 

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For what it's worth, this comes up a lot over at the Explorer forums because the LS is a lot more common in that platform. The general consensus there is that they prefer to have -some- friction modifier on top of the Mobil 1, but not nearly as much as the factory calls for.
 

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Thanks guys! In my Explorer, I went with the 75W140 without any additives, (doesn't have LS). Smooth as butter. But calling for 80W90 and then jumping to 75W140? Just has me a little concerned. And considering the 75W140 is $20+ per quart, I'd hate to waste time, money, and potentially cause damage.

I think I'm jumping on the 75W90 full synthetic train. I might throw an ounce or two of modifier in there, but at any rate, thank you all!
 

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There's no need for friction modifier if you run a synthetic 75w-90 like mobil 1 or Amsoil. There is also no need to run 140, good luck even finding it anymore. Stick with mobil 1 or amsoil 75w available at any parts store, its way better than old fashion 80w90 that need friction modifiers and runs cooler than 140.
Ford switched -to- recommending 75w140 over 75w90. It's definitely not going away. Mobil1's part number for a quart of 140 is 102490, 90 is 104361.
 

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Snap - so really go 75W140? If that's the case, I suddenly care I have a little over half quart left from the Explorer :icon_thumby:
 

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I don't think it matters that much. The 140 will probably run a little quieter, but Ford spent years recommending each. I'd honestly run whichever I could get cheaper.
 

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Last time I changed the rear axle lube in my Sport Trac I used Valvoline Synpower 75w-140. Cost a lot less than Mobil 1. I change it every 30k along with the t-case. I'm told the 8.8 IRS has soft bearings so I'm helping it along. Gen 2 Tracs have a drain plug so it's easy to change anyway.
 

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Ford switched -to- recommending 75w140 over 75w90. It's definitely not going away. Mobil1's part number for a quart of 140 is 102490, 90 is 104361.

I've never been able to find 140 locally. I can order it online but never see it the auto parts stores which always led me to believe it wasnt all that popular. :dntknw:

I just stick with mobil 1 or amsoil 75w90 in every diff I own and never have issues. I tried doing some research but found no real logical answer for the need to use 140 except ford randomly changed its opinion. :dunno:

Both oils are going to behave the exact same way cold but 90 will change less and run thinner hot. Unless ford gave up on good tolerances why would they want to use a heavier oil? I'm not an oil expert by any means but when something has worked for more than half a century someone better give me a good explanation why I need to change. I'm still doing some searching to find that explanation. I will post it if I find something more than conjecture.
 
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Not to hyjack this thread if I go from non synthetic to full sync do I need to worry about contamination? Do you need to flush the old oil out? I need to change the pinion seal and thinking of going with mobile 1.
 

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Not to hyjack this thread if I go from non synthetic to full sync do I need to worry about contamination? Do you need to flush the old oil out? I need to change the pinion seal and thinking of going with mobile 1.
Absolutely not. That whole mixing regular oil with synthetic is an old wives tail.
 

Mike Tonon

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Will 75w90 get me higher fuel mileage than 75w140? If so, how much better?
 

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