1999 F-350, 7.3 diesel, 2 WD 6 speed
The input seal on the differential is seeping and I intend to swap it out.
There are two metal tags on the pumpkin with the following info

W 140 Synth Oil Only

S 409 C
3L73 1058M22

I'm guessing the use of 140 weight synthetic is required. Should I also add the "friction additive" to the oil?

Which is the correct part for the input seal?

Any other pointers or tips given the above identification tags?
Thank you!
Regards,
Eric

 

If you provide the VIN to a Ford dealer parts dept you should get the correct parts. They will also need the axle code and maybe the mfg date from the label on the drivers door.
A few suggestions;
Mark the position /location of the rear u-joint and the pinion flange to assemble exactly as removed. Important to prevent possible driveline problems/vibration.
Then mark the position of the pinion nut on the pinion shaft and the position of the flange in relation to the shaft. You may/may not need a puller to remove pinion flange from splines of pinion. Hammering it off is not recommended. Seal can be removed prying outer ring of seal from housing. Thoroughly clean surfaces and you wll need a piece of tubing/large pipe coupling to match outer portion of seal in order to drive the seal in. Make sure to pack the inner part of the new seal with grease to prevent the garter spring from flying off when you drive the seal in. You need to install the flange in the same position, that is why you need to mark the location and the tightened position of the pinion nut. Assemble as removed making sure to line up all your marks. I believe 75W140 is the lube used. Don't forget to check the level after done, plug on driver's side of differential. If it has not lost much lube , no need to add any additive.
Now is good time to replace u-joint if needed.

 

Per your tag info (3L73), you have a Limited Slip with 3.73 gears.

HANKY is right on the 75W-140 and a synthetic is required.

Being a 1999, I would also go ahead and change out the gear lube and clean the axle housing out real good.

Per the 1999 owners manual-------> Add 236 ml (8 oz.) of additive Friction Modifier C8AZ-19B546-A Ford Specification EST-M2C118-A for complete refill of Traction-Lok axles.

 

Gents,
Thanks very much for the advice. I'll be following it carefully!
Best wishes,
Eric

 

You got it. Let us know how the job works out.