Wanted to change the lube in rear diff in my '92 F-350. Crawled under the truck and found that there is no tag on the cover. Now how do I know what to put in there? Any way to find out if there is a LS or open differential? Puzzled...
Any help will be appreciated.

 

Run the axle code located on drivers side door jamb.

 

If you jack the rear end and turn the drivers side wheel, does the passenger side rotate in the same direction? If so, you have a limited slip, if it turns the opposite direction you have an open differential. Handy trick for the junkyard as well!

 

C9 3.55 Limited Slip
C5 4.10 Limited slip


19 3.55 open
25 4.10 open

 

A '92 F350 has a Sterling 10.25 full floating rear.It takes 3.5 quarts.If you use Mobil 1 LS 75W-140 there is no need to add friction modifiers if you have a LS diff.




Mobil 1™ Syn Gear Lube LS 75W-140 is a high performance, synthetic, multi-purpose, automotive gear lubricant designed to help meet the highest level performance requirements of many modern passenger vehicles in all types of operating conditions, from mild to severe, including limited slip applications, as well as deliver outstanding power transfer performance. Compared to conventional hypoid gear lubricants, Mobil 1 Syn Gear Lube LS 75W-140 performs exceptionally over a wide range of temperatures. Mobil 1 Syn Gear Lube LS 75W-140 achieves this through a unique proprietary combination of synthetic baseoils and a precisely balanced additive system that helps to deliver optimized viscosity-temperature properties together with the highest level of inherent formulation stability. It also helps to protect against thermal and oxidative degradation, wear and corrosion, viscosity loss associated with premature shearing.

My bad,it does state:
Recommended for many limited slip applications. For certain high level LS applications, additional limited slip additive may be required if axle chatter is observed.


Mobil 1™ Syn Gear Lube LS 75W-140



I've read it's best not too add too much or it will just work more like an open diff.Best to add a touch only when using non synthetic and just add as needed to make chatter go away,this will insure she works at turning both wheels for max traction.My guess is,you probably wont be adding anything if you use the synthetic lube.
Of course,if you have an open diff,then this all irrelevant.

 

Wow, that was quick.
Axle code is D5 which in fact would be 4.10 LS but I am the third owner and would not trust the sticker.
Will jack her up tomorrow and spin the wheels as LSCRX suggested. Why would somebody trash the tag...

 

I have C5 on my '93 F-250. Are you positive it's 3.55?
I was looking here https://www.blueovaltrucks.com/tech-...-door-sticker/ and it says C5 is 4.10 and C9 is 3.55.

 

my bad i had it backwards

 

Axle Decoder Chart

Code Ratio Capacity Maker Type Yr

12 2.73 3800 FORD REG 1991
17 3.25 3300 FORD REG 1971
17 3.31 3800 FORD REG 1995
18 3.08 3800 FORD REG 1991
19 3.55 3800 FORD REG 1991
25 4.10. 5300 FORD REG 1991
29 3.55 5300 FORD REG 1991
35 4.10. 6250 FORD REG 1991
36 3.73 7400 DANA REG 1980
39 3.55 6250 FORD REG 1991
45 4.10. 7400 FORD REG 1991
49 3.55 7400 FORD REG 1991
65 4.10. 8250 FORD REG 1991
69 3.55 8250 FORD REG 1991
72 4.63 11000 DANA REG 1991
73 5.13 11000 DANA REG 1991
H5 4.10. 3800 FORD LIMITED SLIP 1991
H7 3.31 3800 FORD LIMITED SLIP 1995
H8 3.08 3800 FORD LIMITED SLIP 1991
H9 3.55 3800 FORD LIMITED SLIP 1991
B5 4.10. 5300 FORD LIMITED SLIP 1991
B9 3.55 5300 FORD LIMITED SLIP 1991
C5 4.10. 6250 FORD LIMITED SLIP 1991
C9 3.55 6250 FORD LIMITED SLIP 1987
D5 4.10. 7400 FORD LIMITED SLIP 1991
F5 4.10. 8250 FORD LIMITED SLIP 1991
W5 4.10 8250 FORD LIMITED SLIP 1995



Transmission codes from 1995 Manual

M = manual 5 speed overdrive (Mazda?)
E = Automatic E40D
G = C6 auto
U = 4R70W (4spd auto with overdrive)
C = manual 5 speed close ratio (Z-F)
W = 5 speed H.D. Overdrive

Engine Codes (8th character in VIN) From 1995 Manual

Y = 4.9
N = 5.0
H = 5.8
R = 5.8 HP
C = 5.9
G = 7.5
F = 7.3 DI Turbo
2 = 4.9 natural gas

Engine Codes From 1991 Manual
Y 4.9
N 5.0
H 5.8
M 7.3 Diesel
G 7.5

 

It's okay. Does happen.

 

Weather is not cooperating but I jacked her up. Spinning wheels on one side makes wheels spin on the other side in the same direction so it's limited slip then. Thanks for the tip LSCRX.
Now found that the fill plug is busted and will need some fighting to be removed... Always something.

 

Yeah whenever you see a letter in the axle code of these trucks it's a LS unit.Good to verify though in case it's been swapped out.
By the way you can run 80/90 or synthetic 75/90 if you want too.Those of use who haul and tow heavy like the thicker (75W-140) lube is all but if she's not a full time worker,you may want the thinner stuff for a bit better fuel economy.Though I dunno if you'd ever notice a difference anyway.

Yeah,you have to fight with something,if it was ever easy where would all the fun be?