Diff. Gear Oil - Additive - Ford Powerstroke Diesel Forum
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post #1 of 6 Old 05-12-2011, 09:08 AM Thread Starter
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Diff. Gear Oil - Additive

I'm ordering some Amsoil 75-90 and Amsoil 75-140 Severe Gear for my F350. I was also going to order a tube of the Slip Lock Additive....but wanted to make sure that this is needed for both the front and rear differentials. The truck has 4.10 limited slip.

Thanks!
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post #2 of 6 Old 05-12-2011, 11:44 AM
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You won't need it in the front. No such thing as a stock limited slip differential in the front of any Ford 4X4.
As far as the LSD in your back diff, You might want to just change the oil and drive it. The limited slip additive balance is the key to effective operation of the limited slip diff. Too much and the clutches will slip when you don't want them to, not enough and you will get "chatter" when turning corners. Don't take anyones word for how much to use if you want the best out of your limited slip.
Change the diff oil, add nothing. Drive the truck for a hundred miles or so (not continuously). After getting the miles on, you know the oil has been all the way through the clutch pack. Now, after running the truck for an hour or so, come to a complete stop. Turn the wheel full lock in one direction and slowly roll into the throttle. You want to be moving between 1-5 MPH. Do you feel the chatter? The truck may actually shake. If you do, to start, add 2 ounces of the additive (by removing the ABS/VSS sensor on top of the diff). Drive it for about 50 miles (or a few days) and try the same thing again. Add only one to two ounces at a time. When you do not have any chatter, your done. Do not add any more.
Many of the synthetic diff oils (Lucas, Mobil1, Royal Purple) do not require the additive, but again, if you feel the chatter and it bothers you, add an ounce or two.
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post #3 of 6 Old 05-12-2011, 12:34 PM Thread Starter
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Quote:
Originally Posted by NYC F-350 View Post
You won't need it in the front. No such thing as a stock limited slip differential in the front of any Ford 4X4.
As far as the LSD in your back diff, You might want to just change the oil and drive it. The limited slip additive balance is the key to effective operation of the limited slip diff. Too much and the clutches will slip when you don't want them to, not enough and you will get "chatter" when turning corners. Don't take anyones word for how much to use if you want the best out of your limited slip.
Change the diff oil, add nothing. Drive the truck for a hundred miles or so (not continuously). After getting the miles on, you know the oil has been all the way through the clutch pack. Now, after running the truck for an hour or so, come to a complete stop. Turn the wheel full lock in one direction and slowly roll into the throttle. You want to be moving between 1-5 MPH. Do you feel the chatter? The truck may actually shake. If you do, to start, add 2 ounces of the additive (by removing the ABS/VSS sensor on top of the diff). Drive it for about 50 miles (or a few days) and try the same thing again. Add only one to two ounces at a time. When you do not have any chatter, your done. Do not add any more.
Many of the synthetic diff oils (Lucas, Mobil1, Royal Purple) do not require the additive, but again, if you feel the chatter and it bothers you, add an ounce or two.
EXCELLENT info. Thank you very much. Like I said....I had no clue about the differentials as I've never taken one apart.

Now as for the diff covers, I understand there is not a gasket --- just some sealant. What is the best to use for this purpose? I do a lot of heavy hauling and don't want a pumpkin leak!

Thank you again.
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post #4 of 6 Old 05-12-2011, 12:50 PM
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Geez, you had to ask. My mind is drawing a blank on the actual term of the stuff. I use the Permatex silicone sealer (blue) in the tube at the autoparts store. I can't believe that I can't think of what it is called....getting old sux.
No gasket required.
Clean the mating surfaces on the diff and the cover. I actually use acetone because it seems to help soften up the goo and it usually wipes right off, no scraping needed.
This might be a good time for you to buy the 08 diff cover, it is finned and aluminum and dissipates heat better.
You want a bead just about half the diameter than a standard pencil. Go around the bolt holes on the cover. I use a matchbook to spread out the sealant evenly.
Then place the cover on the diff, lining up the holes before having them touch. Put the bolts back in. Tighten, fill diff, crack a beer.

One thing you should ALWAYS do before taking the diff cover off:
Remove the fill plug.
If you can't fill it, don't drain it. (although you could use the VSS/ABS sensor hole)
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post #5 of 6 Old 05-12-2011, 12:55 PM
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When I did mine we used the permatex grey stuff...no leaks from it. Good advice too on the additive...I hear all the time people using synthetic lube with the additive already in it, then they go and add in the 4 or 8oz ford recommends and wonder why their lsd doesnt hook.


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post #6 of 6 Old 05-12-2011, 04:35 PM
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I ponied up for the Royal Purple oil with the additive included when I did my '08 diff cover a while back. Works great.
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