What type of gear oil???
 

HI, I own a 1998 Expedition Eddie Bauer. It has the limited slip 3.73 ratio rear axle. It's time fo rme to change the fluid in it. The thing is I don't know what kind to buy. Does it use a special fluid only dealerships have, or is it the cheaper gear oil that Pep-boys sells. I would really apprecaite all your help!!:)

Dealerships will absolutely overcharge for the product.

If you use a standard petroleum 75w90 gear oil, you'll need to add a tube of friction modifier for the LSD, otherwise you'll foam out!

Probably a better choice would be a full synthetic 75w90 or 75w140 (if you do any towing). I prefer Royal Purple (about $8 a quart on the 'net) - it requires no friction modifier. Neither do brands like Amsoil or Redline.

You can find Mobil One gear oil at your local auto parts store - not sure if you need additive with that or not - read the label. But stick with the synthetic - it's better in the heat!

Thanks, I just have one more question. I want to use a gasket this time instead of silicone. What is the name of the axle my expedition has?? Is it the 10.25 sterling. Thanks a Bunch!!

Your expy actually calls for synthetic in the differential so make sure to use it. I had oil express do it for $35 - seemed worth it to me!

Use Motorcraft Synthetic Rear Axle Lubricant F1TZ-19580-B or equivalent meeting Ford specification WSL-M2C192-A.

The friction modifier additive for limited-slip diffs is Ford Additive Friction Modifier C8AZ-19B546-A or equivalent meeting Ford specification EST-M2C118-A.

Put the friction modifier in first, then fill the rear diff (whether it's the 8.8" or 9.75") to a level 6.4 - 14.3 mm (1/4 - 9/16 inch) below the bottom of the fill hole.

AFAIK, for the LSD, Ford calls for 75w120 + friction mod. I haven't seen 75w120 anywhere but at the dealership (figures) and it's like $30/Qt.

Redline makes an 80w120 syn lube with goes for about $8/QT. I'm probably going to go for that - Redline is a great company.

I think the usual axle for these trucks is the 8.8, unless I'm mistaken.

XXL

I think the "rule of dumb" (if not 100%) is 8.8" for the 4.6L and 9.75" for the 5.4L. The front differentials are 8.8's".

OK, I just looked in my manual, the rear differential calls for 75W-140 and the front differential calls for 75W-90 (Plus friction modifier if LS).

FYI, the 9.75" rear has 12 bolts on the cover and the 8.8" has 10 bolts.

i have 98 expy there is a tag on my diff. that says to use 75w140 synthetic so i added fords additive and i bought redlines 75w 140

also my axel is a locker

The shape (outline) of the diff covers is not the same. The 8.8 diff cover is pretty much symmetrical when comparing the right side to the left side, while the 9.75 cover is relatively rounded on the left side and somewhat triangular-looking on the right.

i thought that the fluid in the rear was good for life unless there were signs of a leak?

If you take the truck for a little bath in a river or stream, you may need to change it (water contamination). Never hurts to change it anyway to flush out metal particles from normal wear.

Depends on your definition of "good for life" - Ford seems to plan for people not keeping these trucks past 120K miles or so.

As PaulC said, there's no good reason not to flush the rear differential - cheap insurance for a costly failure.

XXL

My rear diff uses 75w140, BTW, not 75w120 as previously stated. I haven't dealt with this in over a year - memory isn't that good :)

I'm going to peek under there tomorrow - I can't remember which rear I've got. I thought it was an 8.8", but who knows....

I certainly don't ;)

XXL

I think the ford maintenance guide says every 60k miles. I had a dodge dakota for about 150k miles and never changed it, never had a problem with it - different animal though!