I have a 2004 Grand Marquis that I'm having the differential fluid changed on. I'm having Redline gear oil 75w-90 put in. My question is this....the lube comes with a friction modifier in the blend but says more MAY be needed. I purchased the extra friction modifier in a 4 ounce size. (My vehicle has the traction-lok). Do I need to add the extra modifier or will it be fine with what's in the fluid already??

Here is what the lube guide I use recommends, and it only covers vehicles from 96-06.

link http://www.supermotors.net/getfile/700942/original/penzoil-demostic-lube-guide-9606-diff.jpg



02-06 panthers with limited slip need 4 oz. of friction modifier total.

Type of fluid- synthetic 75w-140 (not 75w-90)

The capacity of 04-06 panther rear end is 4.6 pints. 96-03's take 3.8pts

And last thing 96-06... fill to 6-13mm bellow fill hole.

Answer to your question... It depends how much modifier is in the gear fluid you have. (you need 4oz total)

Hope this helps you

75w-140?? This is what my manual says....
Rear Axle
Lubricant
Motorcraft SAE
80W-90
Premium Rear
Axle Lubricant
XY-80W90-QL WSP-M2C197-A

I'm really getting confused now. Why the 140 weight all of a sudden?

I'm not sure, but I'll tell you this, When I first got my car I put 75w-90 in the rear-end and it made a lot of noise untill a couple of weeks ago when I switched to 85w-140... and now no noise. The noise was a constant louder than normal whining from what I think was the pinion. I don't know if it would do any damage to run the thicker fluid but here is an interesting side note, 97+ limo's with limited slip also take 75-140

You will have to drive it afterwards and see if it's grabbing too much/grinding or chattering on tight turns. If so, put half the bottle of FM in and see if that helps.

If the traction lok is just being installed I'd dump the whole bottle in. The clutches most likely haven't been soaking in FM.

80w-90 is spec'ed by the company that actually made the car 75w-90 will be fine.
You should notice NO increased noise using it, keep in mind the factory fill was also 80w90.


75w-140 is for the heavy duty applications which is why it is listed for Limos and later CVPIs.
You may notice a slight gas mileage hit with 75w-140

I read in Police fleet magazine that they specify 75w-140 SYNTHETIC gear oil for ALL CVPI's (yes they back revved or back dated this)

You don't have a police model but if you are hard on your car It's probably a good idea.

I use the AMSOIL 75w-110 Full Synthetic gear oil - a touch thinner (more power and better mileage albeit very slight) and no need to add any FM (it's in it already)


Ok is that enough parenthesis for ya?

Ok...you've convinced me to use a slightly heavier fluid. I ordered some Amsoil 75w-110. But are you sure it has the friction modifier in it? I'll have to go back to the website and take a look.

Out of curiosity how much friction modifier is usually needed when a trac-lok unit is first installed?

I'm not sure but in the manual they just say to put in 4oz. when changing it, if the new fluid does not have any modifier.

Yes, I'm sure it's in there.

How about a nice Trick Flow Girdle (pictured left) while you're at it?

The Mustangs get filled with synthetic 75W-140 from the factory.

OMG! I want that trickflow cover! Where'd you get it! BTW I got that ring and pinion too.

Do they sell 2.73 rear gears or even lower gears?

Click my black Bronco in my sig & look in the 8.8" Axle album.You can get a similar stock one in the JY from certain Explorers & Expeditions. It'll have some big mounting ears that you MAY need to cut off.