For the LS, it states that synthetic is required. For a conventional diff, you can use the regular gear oil.

You can use any synthetic as long as it meets the same specs. Go to Napa, O'Reilly, AutoZone, Advance, etc., it's a lot cheaper. To the tune of about $9 a qt vs. $17.

 

yes i know that it is the synthetic oil but what is this additive that everone is talking about and they are saying only add 2oz. at a time that is not in owners manual

 

these guys are talking from experience when talking about the friction modifier.if theres too much in there it lets the clutches slip too easily.thats why they are saying to only add a little at a time,just enough to get the clutches to quit chattering.this will get the diff to perform as good as it can,which still isn't that great.

 

It's just an additive. It usually comes in a 8 oz tube. You can usually find it right near the rest of the lubes.

Look in the maintenance tables in your manual, it shows it in there.

Friction Modifiers
In an effort to control this chatter, friction modifiers are often used. These additives result in a smoother transition between the two states of friction: static and dynamic. Static friction occurs when the clutches physically lock together, while dynamic friction occurs as the clutches are slipping.

 

My 99 f350 LS only works after I back up a couple fo feet. Do you guys think a fluid change will make it work better? Thanks for all of your help.

 

1 - Have you ever changed the fluids before?

If not, read a few of the posts above about ahow much to add at a time.

Ford says 8 oz right off the bat. That's usually too much. Change the fluid with no modifier and drive to a big open parking lot (Sundays are good). Do a series of sharp turns. If the LS unit is working, you'll hear and feel it trying to grab. If not, you may need to replace / rebuild. If you do hear the chattering, add 2 oz of the modifier and repeat the series of turns. You want to do this unit the cahtter can just barely be felt / heard under a full turn on a hard surface.

2 - How many miles on the truck? Reason I ask is that clutches on the diff wear out like clutches on the trans.

A fluid change would be the first course of action. Rebuilding or replacing the LS unit would be next.

 

It Has 130K on it.I got the truck used and it has chrome dif covers on it.It is pos the wrong fluid was used to erfill them.I will change the fluid and let you guys know what happens.Thanks again

 

You can buy the modifier at Advance and other stores too, I'm sure. The brand they had that I picked up yesterday is called "Trans-X" posi-trac limited slip gear oil additive. I'm not putting any in until I see how much chatter(if any) I get. Of course it's raining like crazy here now so I don't know if I can test it out or not. Sounds like an excuse to go try to spin the tires in an empty parking lot to me. That's not something I usually do, but I don't want to tear up my lawn again either...

Wal-Mart has Super Tech synthetic gear oil for about $7 a quart but they didn't have 80W90 when I went in, only the 75W140. I would have liked to get it in a gallon jug, but they only had quarts on the shelf. They did have Rotella T 15W40 dino oil in a five gallon size and Jeremy the automotive Dept. Mgr. is going to check on getting Rotella synthetic 5W40 in the bucket for me tomorrow when everyone's "back to the salt mines". BTW, the Rotella T was about $46 for 5 gallon bucket, Rotella synthetic just under $16 for 1 gallon jugs.

Mobil 1 says the modifier is not needed, guess I'll find out soon enough. Doesn't really matter though, I'll be changing fluid again when I install the Mag-Hytec differential covers I ordered. I'll have to get a selector switch for the tranny temp gage and temp probes for the two differentials, then I can check any one of the three when the gages come in and I get them installed.

 

That's correct! Some of them already have the modifier in them. Check the manufacturers site, they will tell you whether you need it or not.

I know Amsoil and Redline synthetics do not require the additives. Just the two I'm familiar with.

 

i used mobile 1.in hindsight,i think i should have gotten some synthetic without the modifier in it,so i could have added just enough to get it to quit chattering.it's possible with the sensitivity of the stock lsd that the gear oils with the modifier in already may have too much.

 

Yes, the stock LS seems to be very sensitive in how much modifier is in it and your statement is on point. I would try that first before you invest in the locker. I guess I was lucky that mine worked correctly right from the factory. However I am sure that all the tight turns from plowing is what did in my diff.

 

In hopes of trying to improve my factory LS (and I am changing out my diff cover with a finned aluminum), I am going to do what most suggested (refill with 75W140 and add 2 oz of friction modifier at a time until chatter is gone). So, here is my question: What 75W140 syn should I use? I have some Royal Purple left over from another truck I owned however it states that it already contains friction modifier so I guess I don't want to use that....right? I have seen Mobil1 in 75W140....does that also contain friction modifier already? Any suggestions appreciated....thanks.

 

Use any quality synthetic oil of your choice, but make sure it does not have any friction modifier in it. It should state if it does or does not on the bottle. Fill your diff and then add 2 oz of modifier at a time until you get it where you like it. That should do it!!

 

i believe,and i'm sure i will be corrected if wrong,that the motorcraft lube doesn't have modifier in it already.it's pricey compared to other stuff out there but i wish i had gone this route and only added JUST enough modifier to get it to quit chattering.

75w140 and 20/20 hindsight.

 

Thanks for the replies guys! I could only find 2 75W150 syn available. Motorcraft ($23.50/quart) and Red Line ($8.99/quart Model 75W140NS). So, I have ordered the Red Line and will change it out next week. We will see what happens!