What's the additive to put in the rear to keep the clutches from slipping and where can I buy?

Do I have to buy ford or can I buy some at oreillys?

 

Are you replacing the fluid? Or, just wanting to add a friction modifier because you are hearing something?

From what I have read, auto parts stores do sell a friction modifier, so you don't have to depend on Ford.

If you are changing fluid altogether, use Mobile 1, or Valvoline LS. Both already have it in there. (Make sure you read the bottles)

 

Ditto, the Mobil 1 LS fluid is good to go right out of the bottle, no need to mix additive, although you will need another auto loan to buy the stuff

 

I bought Valvoline Synpower Full Synthetic 75W-140 for the rear differential. But I got to thinking about the friction modifier so I called Valvoline to ask them if I needed to add any to their product. The guy I talked to knew about Fords and said to add 4 oz. to the Valvoline. I asked him if I should try without the modifier first and he said absolutely not. It's only a couple of bucks for the bottle so I picked it up at the Ford house.

In a couple of weeks when the weather gets better, I'll climb under the truck and do the job. It's a shame Ford didn't put a drain plug in the cover - would be a lot easier to change. The differential on my truck is the 10" Sterling differential. It you have another differential, you may have other requirements.

 

You just use the proper viscosity as called for in your manual and that is it... the only time you "add" the extra add-pack/modifier is if one has the "limited slip differential" option on their truck.

I have watched and asked my local dealer do the work on my truck and the rear-diff has a metal tag on it and/or they can look it up by your VIN#... but my 2003 Excursion 4X4 (non-limited slip) has never needed more than then just the proper rear-end fluid called for by Ford.

 

Wonder if he knows what their literature says? (Unless you and he were talking a product that doesn't specify Limited Slip? )

http://www.valvoline.com/pdf/synpower_gear.pdf

"VALVOLINE SYNPOWER FULL SYNTHETIC GEAR OIL W/LIMITED SLIP
Valvoline SynPower Gear Oil is a superior sulfur-phosphorus extreme pressure gear lubricant formulated with synthetic basestocks and additives to provide excellent performance. It is designed to provide excellent extreme pressure protection, load carrying capacity, anti-foam performance, corrosion protection, and thermal stability protection. It is recommended for conventional and high performance applications. Valvoline SynPower Gear Oil is recommended for use in axle applications requiring factory-fill or drain-and-fill levels of limited slip performance.
Valvoline SynPower Gear Oil is recommended for use in axle applications requiring factory-fill or drain-and-fill levels of limited slip performance. Valvoline SynPower 75W-140 is also recommended for use where Ford M2C-192A, GM 12346140, Chrysler MS-8985 , or GL-5 SAE 75W-140 gear oil is specified. The addition of a supplemental friction modifier(Ford M2C-118-A, Chrysler MS-5630, or GM 1052358) is not required."

 

I put a Mag-Hytec diff. cover on, used Mobil 1 Syn & still had to add the modifier. Without adding the modifier it would ratchet when turning.

 

You are probably the first I have read that needed more with Mobile 1.

Thanks for the heads up!

 

After researching the Mobile 1 I didn't think Iwas going to need the modifier either. But it wasn't just once in a while occurrance it was ratcheting on every right or left turn. Even backing up & turning.

 

"Wonder if he knows what their literature says? (Unless you and he were talking a product that doesn't specify Limited Slip? )"

The Valvoline rep I talked to knew that their product was LS and their documentation said no additional modifier was needed. He then pointed out that worked OK in everything but a Ford F250/350 LS. He said to add 4 oz. For lubricants that do not include friction modifiers, Ford says to use 8 oz. I'll take the Valvoline rep at his word and put 4 oz. in.

 

Here's the web page for Mobil 1 75w-140 LS Synthetic Gear Lubricant. & you will note that's it says ". . . including limited slip applications . . ."

Gear Lube | Mobil 1™ Synthetic Gear Lube LS 75W-140

 

When you say "ratcheting" was it a series of "ratches (?), I am getting a one time thunk-clunk... am I having the same problem, or maybe Ive got something else going on?

 

OK I've got to show my age. I factory ordered a 1965 Corvette, 396 CI, 425hp fastback with positraction (Limited Slip) in April 1965. Picked up in June 65 & right out of the box so to speak the rear end would have a deep tone click (clunk) when turning. They said it was the positraction & they needed to put someting in the gear lube. After that everything was fine & they, back then said it was ratcheting. When my F350 was brand new & I told them it was ratcheting they knew exactly what I was talking about & added more modifier. The up side was the service manager was an old timer like me.

It' usually more that just one clunk. When in a tight turn it would be a clunk, clunk, clunk as you make the right or left turn.

 

Interesting thread. I used the Mobil 1 fluid for limited slip differentials and I did NOT use any additive.

I am NOT noticing ANY issues with it.

 

When I redid my rear diff fluid I read all the threads and the mentions about to add or not to add the additive.

I choose the Mobil 1 product because it was mentioned so often in relation to the product that the additive wouldn't be required.

When I went around the block after I did the fluid swap, on one tight corner I did get some chattering definitely from the rear end that felt the way the front end feels when you have 4x4 engaged on dry pavement. I thought to myself "damn, now I got to research what this additive product is, where to get it, how much to add, why me if it doesn't happen to anyone else, just my luck".

Next morning driving to work at the same corner no chatter. Hasn't happened since and I haven't worried about it. That's my story.

That was Oct 2011. 43,000 miles ago.