F 150 2WD Pickup V8-5.4L Prop SOHC VIN Z (1998)
Fluid - Differential: Fluid Type Specifications
The rear differential fluid is the same for the 8.8, the 9.75 and the 10.25 inch ring gear:
Motorcraft Synthetic Rear Axle Lubricant part number F1TZ-19580-B.
Meeting Ford specification WSL-M2C192-A.
This is also called SAE 75W-140 High Performance Rear Axle Lubricant.
With additive:
Ford Friction Modifier part number C8AZ-19B546-A.
Meeting Ford specification EST-M2C118-A.
