Limited slip additive?
 

I have an FX4 with a limited slip differential. I want to change my gear oil because I'm worried that I might have been in water up past the top of the vent tube. Before I realized that I needed to add the Ford limited slip additive, I went and bought some gear oil. This is the stuff I bought:

Attachment 116985

It says limited slip on it so this is my question.. Does this contain a suitable additive? If not, is it okay to use with the additive or should I buy regular non-limited slip oil and then the proper additive? Finally, do I have to go to Ford for the additive or is available at parts stores? Thanks.

Get the CRC LS Additive... Though it says it works with LS carriers on the bottle, I wouldn't trust it has a true slip additive in it. Spend the $8 on the tube at add it in after you add the 1st quart. then continue adding your fluid til it comes out the fill plug.

Generally if it is a limited slip oil it contains the additive.
Personally, I would use that stuff and see how it works. You can always add a bottle of additive later if the differential is too tight and chattering. I would rather have too little than too much when it comes to a limited slip additive, but that's me.

I always used Lucas in the ranger, it has the additive. and I'm running Royal Purple (which has the additive) in my Z71s rear axle.. the LS seems to be functioning properly.

On the other hand, the posi 9" 3rd member I had for the Ranger. I put in the cheapie gear oil and the ford additive.. and nothing.

Okay thanks guys!

Per Ford specs, if your LS is a Torsen, then you MUST use 75W140 synthetic gear oil plus the friction modifier.

My owners manual says 80W-90, and 75W-140 synthetic for FX4 Level 2 only?

Correct; the FX4 Level II is the model that comes with the Torsen limited slip. The regular FX4 comes with a clutch-pack type of limited slip.

No need for additive in a Torsen, that's only for clutch pack type LS. You do need to run the syn 75-140 though.

That's weird, I'm running 75w-90 in my L2 rear end with the CRC LS additive, no problems... :)

IIRC, Rick over at ORR who worked on developing the Torsen for the LII stated that the call for 75-140 was based on the 31 spline axle. It did not NEED the 75-140, but it was carried over from the Ex axles that were under harsher conditions, i.e. V8s, higher tow rating, etc. That's all what I remember, could be wrong a few places...

I'll agree that it sounds weird, but the official Ford shop manual calls for the additive in numerous places in the section dealing with the servicing of the Torsen rear end (Section 205-02C). I'm one to believe that Ford's engineers know a lot more than me. I quote (italics mine):