2001 Ford E450 Rear Diff.

As stated it is 75-140 synthetic oil. The diff cover should not have a gasket but rather RTV sealant from the factory. There is a plug on the side which is the full mark and can be used to add fluid as well. If you remove the cover you will need 3 quarts and a tube of posi additive to refill. Piece of advice....be sure the cover is not rusting through causing the leak. Ford had issues with this until 07 when the cover became aluminum....I know cause mine let go resulting in a total diff replacement. A new cover for 30 bucks is well worth the investment if changing the gasket

^^^ It's a Ford Chassis? If so, there's a Vehicle Speed Sensor (VSS) on the front top of the differential. Held down by a 10mm capscrew, sealed with an O-ring. Clean the area, pull the VSS, and pour your new lube down the VSS hole using a small funnel. Do this with the magnetic fluid level plug removed. Stop when the new fluid reaches the level plug hole.

How do you know it's full? I have a leak and want to put in a new gasket.
Thanks

Motorcraft® SAE 75W-140 Synthetic Rear Axle Lubricant

https://www.fcsdchemicalsandlubricants.com/main/product.asp?product=SAE%2075W-140%20Synthetic%20Rear%20Axle%20Lubricant&category=Transaxle%20Fluid

k9-keno wrote:
What fluid goes into the rear pumpkin/Diff?? Is it gear oil and if so what kind?? Thanks

I don't know what goes into the newer ones, but it used to be a 90 weight gear oil, or an 80W90. I'm assuming is would still be the same thing.