Rear Diff fluid change questions

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1955moose

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I'm a little confused. In your first post you stated you purchased the correct motorcraft oil, and 4 oz of additive. Then you stated you put in Mobil 1 75-90 oil that you'll be changing! Why didn't you use the Ford oil you bought at dealer?

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UncleDJ

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With 68k on my 14 EL, I can't say that it's ever been changed so I'll gi-er-dun. I'll go full synthetic and go with friction modifier, but don't know what weight and how much.
Any suggestions appreciated.
Thanks a bunch. :)
 
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Vankonk

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1955moose.....good catch! I screwed up and never looked at how much oil I needed. I only bought 1 quart of motorcraft and needed two. Didn’t realize I was screwed until I drained the diff and saw how much came out. It was late and only thing open was advanced auto so went there and bought what they had.
 

16plati

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Will be performing rear diff fluid change on my 16 to fix the same issue, vibration and rumbling at low speeds after turning left or right from a stop
 

16plati

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Glad fluid fixed the issue.
Sounds like they did not add friction modifier either.

I just hit 47k so I’ll be doing my differentials and transfer case at the end of the summer.
Which will be good just in time for winter when my 4wd gets used most.
Did you use the owners manual recommended SAE 75W85? I see OP used 75w90.... what should be used?
 

16plati

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It stays in 2H all the time unless I need 4auto for snow.

I did drain it last night and I think the previous owner or dealership either used the wrong fluid or it was severely broken down. It ran out like water. I was expecting it to pour out but not like that. It was empty in a matter of seconds. It was dark and there was no signs of water contamination, just very runny. Also when I pulled the fill plug up top, fluid started to leak out of the fill. One other observation, was both the fill plug and drain plug had some type of grease covering the magnets. Not sure if that was normal or not. So a couple of strange things.

Changing the fluid fixed the problem. No more shutter/vibrations in slow turns. I ended up using Mobil1 75W90 full syn with the friction modifier. I know that's not the recommended oil so I might get some amsoil and do it again. Like you said it doesn't take long and not that expensive. I didn't even need to jack up the truck to do it.
Tht “grease “ is apparently minute pieces of metal. Normal for wear in period. You do not want to see chunks of metal in it
 

Black

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I used Amsoil 75-90 Severe Gear Oil I do not believe Amsoil makes a 75-85.

I used Ford Motorcraft Friction Modifier.
I believe Amsoil is call Sliploc I have never used it as strangely no one carries it locally.
 

16plati

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I used Amsoil 75-90 Severe Gear Oil I do not believe Amsoil makes a 75-85.

I used Ford Motorcraft Friction Modifier.
I believe Amsoil is call Sliploc I have never used it as strangely no one carries it locally.
Awesome, did you just pull the drain plug or use fluid extractor? I’m trying to keep this as cheap as possible so pulling plug and letting it drain seems to be easy enough
 
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