92 F-350 Diff fluid change

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32
Location
MI
I have a 92 F-350 with the 10.25 rear end and am wanting to change the diff fluid and was wondering if i was to use mobile 1 75w90ls do i need to add the ford friction modifiers? I am also open to suggestions on diff fluid. The truck is a chassis cab with a 8X12 dump bed and is used to haul fire wood. If currently has 28,000 miles and the manual says to change it at 30k for severe duty.
 

JHZR2

Staff member
Messages
46,257
Location
New Jersey
Likely no need to add the friction modifier, especially since M1 already has it. If it chatters, just add some sparingly. At only 28k on a 93 truck, id be worried about other things. IMO M1 is fine but if you haul heavy a lot, 75w-140 may be better...
 
Messages
26
Location
Texas
I run syn 75-90 (amsoil) in my 95 F-350 and have been happy. I would just run the M1 without any additives. Just a note some newer trucks with this diff spec 75-140 but either one would work fine.
 
Messages
6,990
Location
Everett, Washington
Amsoil site shows 90, or 110 wt for the gear lube for a diesel. 110 wt is an Amsoil wt. So the 90 should work fine. Amsoil says you can run the gear lube and if it chatters add friction modifier. If it was me I would just add it at the same time.
 
Messages
4,564
Location
NW Ohio
You only need friction modifier if your truck has a limited slip (LS) and but if it does, the synthetic oil may have the required friction characteristic with no additive needed. Also, some oils are sold with the right amount of FM already added (and they say on the jug). Overall, you want to avoid adding it if you can, as the FMs tend to reduce the oxidative resistance of the oil. If your truck has a LS, put in the syn oil without a modifier, drive a few Figure-8s in a parking lot (which works the oil up into the clutches) and drive for a few days to see. If no chatter, great... go on about your biz. If chatter, add FM an ounce at a time, doing the parking lot thing between until the chatter stops or you reach the maximum amount listed for that axle (3 oz according to my book). If it does have a LS, you will find a tag on the diff that has a number that indicates the axle ratio and the presence of a limited slip. For you F-350, the axle ratio is likely 3.73 or 4.10:1, so the tag might read "3L73" or "3.73L" if it has a limited slip with a 3.73 ratio and without the "L" whatever ratio without a limited slip. Your door sticker will also have the axle code under "Axle" or "Ax". Rather than list al the codes, if you want to know, go back and look and put the code back here and I'll look it up next time I'm here.
 

JHZR2

Staff member
Messages
46,257
Location
New Jersey
Originally Posted By: oilboy123
Amsoil site shows 90, or 110 wt for the gear lube for a diesel. 110 wt is an Amsoil wt. So the 90 should work fine. Amsoil says you can run the gear lube and if it chatters add friction modifier. If it was me I would just add it at the same time.
I forgot about the 110 wt. Actuall, to the OP, Id try and run a 110, which is the heavier end of what used to be a wider 90wt viscosity range (remember that a single value viscosity is actually a range). I like the idea of a 75w-110 SVG if the truck is specced for 75w-90, given the hauling.
 
Messages
4,564
Location
NW Ohio
Originally Posted By: zr900
No not pints the manual calls for 7.5 quarts.
Sorry, you must have misread. It's 7.5 Pints. I have two trucks with the same axle. Plus I have factory manuals for '85, '86, 87. '89. 90, 91, '96, '98, 00 and later. With minor variations, they all say 7.5 pints. If you don't believe me, by all means buy 7.5 QUARTS and try to put it in
 
Messages
908
Location
Tornado Alley
1992 F350 Differential, Dana 44 Front..........3.6 pints [2] Differential, Dana 50 Front..........3.9 pints [2] Differential, Dana 60 Front..........6 pints [2] Differential, Dana 60-IU Rear..........6.4 pints [2] Differential, Dana 70-2U Rear..........6.6 pints [2] Differential, Dana 70 HD Rear..........7.4 pints [2] Differential, Dana 80 Rear..........8.5 pints Differential, Ford 10.25 RingGear Rear..........7.4 pints [2] Differential, With 8.8 or 9 RingGear Rear..........5.5 pints [2] Transfer Case, Warner 1345..........6.4 pints Transfer Case, Warner 1356..........4.1 pints My 10.50 holds 3 3/4 quarts... Just changed it last spring... What ever gear oil you chose...Just drain it and add new around the 30K to 50K mile mark... If you tow often and heavy... I would not go past 30K miles... It's always better to use a thicker viscosity in gear oil... It my hurt your mpg a bit... but adds better lubrication as well as protects against heat better... Redline and Amsoil and M1 and Schaeffers are all top performers and can withstand many miles of abuse...
 
Messages
4,564
Location
NW Ohio
sr900- Then your manual is a misprint or a mistake. Please, this is not the hill you want to die on. Check another book. Having changed the diff oil in my trucks numerous times (one an '86 that I have owned since new) I assure you that if you indeed have a 10.25 axle, buy 4 quarts and fill to the bottom of the fill plug on level ground, you will have some oil left in the 4th quart. I have Mag-Hytec covers on both my 10.25s and even then, they only hold 6 quarts.
 
Messages
908
Location
Tornado Alley
zr900: I had time to look into the specs of the SuperSyn gear oil... From the specs and what I read... I'll bet it as well will be a top performer... Good Choice!!
 

zr900

Thread starter
Messages
32
Location
MI
I went with the 75w140LS Smitttys SuperSyn. At only $6 a quart and the good luck we have had at work with their hydro oil I figured I would give it a try.
 
Messages
4,564
Location
NW Ohio
Keep us posted on how it works out for you! I don't think that oil has every been discussed here before.
 
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