1989 F350, 460, 4.10 rear, pos (limited slip differential), C6

The differential fluid was never changed. Should I change it? I also know that due to posi it requires a special type of fluid, not just any gear oil.

How does it get changed?

What's the best oil to use?

 

Pull the fill plug - there should be a magnet embedded in it. If there are filings on it, you need to change the oil. Dip your finger into the oil and see how it looks - if it's very dark or milky, you need to change it.

Otherwise, there's no need.

If you do change it, any decent 80-W90 gear lube will work. With that many miles on the LSD clutches, they're probably worn out anyway, so it's not important to get the additive in the oil. You can buy it separately or buy gear oil with it already mixed in.

Unless you pull the rear cover, you have to use a suction pump to drain it.

 

I found out it takes 7.5 pints or 3.25 qt.

What's a good synthetic gear oil to use - dino vs synthetic?

Do I need to use a gasket or just RTV stuff?

 

i have always just used rtv to reseal the cover, applied to the cover only. i prefer to remove the cover to change out fluid, you can clean all interior surfaces and inspect the unit for wear/damage while you're at it. as far as brands, i've heard here Redline is the best out there, i put amsoil in mine because i abuse this truck thoroughly, but it's $30/gallon. make sure what weight/type is recommended, owner's handbook? i changed out my rear lube w/out looking first, but in the '97 newer 8.8 only 85w140(?) synthetic is to be used. yours probably doesn't care.

 

I use either Mobil 1 or Valvoline syn plus Stalube limited slip additive(Meets Ford and Chevy spec). You can tell if you need additive, your rear end will "chatter" when doing very slow tight turns in a parking lot, especially in reverse it seems.

I prefer to pull the cover and do a inspection of the gears and anything else I can see inside. Also allows you to flip the last dregs of any sludge out from the bottom of the diff. I would recommend changing fluid at least every 50K miles or probably every 5 to 10 years whichever comes first. If you drive thru deep water I would recommend a change if your axle was under water for more than a minute or so. A minute might be pushing it.

I have used gasket sealers and gaskets and so far I prefer using gaskets even if the manual says use just sealer.

Good Luck,

Jim Henderson

 

As far as I know, this truck has never seen water. But the diff fluid is 14 years old even with low miles for its age.

FWIW, I looked in both owner's manual and FSM and they don't tell the weight, just the part number.

I want to use a gasket - must I also use RTV on it?

 

Silicone (RTV) is better for several reasons. A Haynes manual will answer all your questions. Use "gear lube"; 80W90.

 

I used RTV before, and when it dries, it forms essentially a gasket. This is what you mean, it is good enough.

 

I got Mobil 1 synthetic 75W90 gear oil. Is that OK?

I also got 2 bottles of additive from Autozone, not sure what it is.

 

Mobil 1 syn 75W90 with additive will work fine. That is what I use in all my diffs. Also a chebby website recommends this combo for the LS rear end. This combo, especially syn should be good for just about any temperature condition you will see in the continental US and works fine for towing in hot temps and cold. I am not sure of the temp range for dino.

Note that you will probably need two tubes of additive. First off because the diff probably has the capacity for it and also because the chebby site experience is that there is less chatter with two tubes, 8oz each. Start with what the manual says and add more if you get chattering.

Good Luck,

Jim Henderson

 

I got 2 tubes of "Trans-X posi-trac" limited slip additive from Autozone. It says it meets Ford #m2c116-A recommendations. Is this stuff good or should I get something else? I hear it has some issues, where do I get the good stuff?

P.S. Looking closely at the Mobil 1 gear oil, it says: Eases gear shifting and prevents chatter in limited slip differentials . Given that, do I need any additive at all?

Is this true?

 

I decided to get some OEM additive, went to Ford dealership and got 2 4-oz bottles of Motorcraft posi friction additive. Is 8 oz total enough?

 

A couple of years ago, my BII with limited slip rear axle was making some noises like a gear whine in the rear axle. So, I pulled the cover off and cleaned 16 years of gunk out of the bottom, RTVed the cover and refilled with Valvoline 80w/90 and a couple of the small bottles of additive from the Ford dealer as all I was hearing from the parts houses that the dealer was the only place I could get the additive. It all went back together well and I still had the whine. I just drove it around easy and found out a year later that my rear driveshaft (junkyard part a few years before) was bad.
Got the rear differential cleaned out anyway. Although getting the new oil in was quite the trick. I quickly tired of squeezing the bottle, so I got some clear plastic tubing that fit onto the bottle and set it up like an IV bottle hanging from the luggage rack on the roof on down under the BII to the fill hole in the differential. Worked pretty good!

*****

 

I am going to take the cover off tomorrow. Would be interesting to see the condition of the fluid.

 

A tip or two...

An easy way to get fluid back in the diff is to buy one of those pump thingies that screw onto the top of the bottle. Then you can hand pump the fluid into the diff. Much easier than squeezing the bottle. Thank god for 75W90 fluid, much easier to pump than the old 90W140.

Once the diff is full, take your truck and do some figure 8s in a parking lot. This helps stir things up and distribute the fluid out to the ends of the axles. May be worth rechecking fluid level later and making sure you have no leaks.

Good Luck,

Jim Henderson