The owners manual wont tell you if it is lsd, check your door jamb sticker for the axle code, What is my Ford rear end ratio, do I have positraction - Drivetrain.com

 

I think I got 5 quarts, but I could be wrong.

 

If your seat is original look behind it in the metal web that holds the seat cushion. Your RPO list may be there, unless somebody took it out. It will list the RPO's "Regular Production Options". It is the build list for your truck. They can be kinda hard to read sometimes. They will be 3 characters. For example a 99F was a 7.3 powerstroke a 99P is a 6 liter version. Hundreds of codes. May help. or call the dealer. The additive wouldn't matter if you didn't have a limited slip. It is for the break disks in the diff. It does matter. I fixed a diff in a 550 last year and I forgot about. I don't do little rear ends much and didn't think about it and it started making noise on turns. Put the additive in it and it quit instantly. Like magic.

 

Sometimes you will see oil amounts listed in lbs. 1 lb = 1 pint.

 

The manual says to fill it till it starts coming back out the hole, then just screw the plug back in. Thats on a 10.25, I'm not sure if thats what you have though.

 

This stuff.

 

On mine the build sheet (as any people call it) was under the seat in the metal web. It was also pretty well destroyed by being there for 20 some years with the seat flexing. Between the flexing and some moisture rusting the wires, it was almost falling apart when I pulled it out. I'm not sure it if could be removed with the seat in the truck either, at least not in one piece.

 

Here ya go.

https://www.ford-trucks.com/forums/9...ur-trucks.html

 

I run royal purple in all my drivetrain except motor oil Trans,diffs and love the stuff its costly though

 

They didn't have the synthetic stuff that I wanted to get so I just got some Valvoline regular 80-90 and a bottle of LSD fluid. I wish I had seen the comments here before I started this morning. My axle code is 39, so I do not have a LSD. The oil probably didn't need to be changed because it was only a dark brown. I have seen black oil come out of these things. I bought a gallon jug and the additive was 7oz I put all of it in. The amount of oil that came out was only about 3/4 gallon. I also bought a felpro gasket and decided not to RTV seal it. No leaks so far, we'll see after the test drive.

I didn't see a conventional fill plug so I unbolted and removed what looked like a speed sensor and put the oil in there. This was on top of the pumpkin towards the front. A real Bi** to get to.

 

Mine has a plug on drivers side right beside driveshaft on front of pumpkin

 

you can't properly fill it there though.
the fill plug is there.its likely just covered in grease or something.

 

The fill plug is on the side on the axle facing the engine. A 3/8 breaker bar fits into the plug. If you overfilled it, remove the drain plug and wait till it stop flowing out and screw the plug back in. Then it should be around the right level.

 

I took another look and it was right where you said it was. To make sure I did not overfill the differential, I opened the plug. About a cup of oil came out. I cleaned the plug and put it back in. I should be all set now. I drove a few miles, so I will check for leaks a little bit later.

 

have a small leak at the rear diff on an 85 e350 van that i was going to reseal today. at the autoparts store i had them look up the fluid (prior to this post) and they gave me 80-90 and a 7oz container of LSD oil additive. the gear oil they gave me is not synthetic though. that ok? seems like everyone on here was using synthetic. don't really know the pros and cons so thought i'd ask...
thanks