i have a 99 ford f350 lariat 4x4 i have to change the front seal on my rear diff. i know what kind of gear oil to use the 75w140 synth i plan on using the royal purple my question is how much do i need i want to completly change the fluid. my other question is what rearend do i have there is a tag on the rearend that reads 3 S 409 E
L73 10 5 9F24

any help will be greatly appriated thanks.

 

The "L73" would indicate that you have a 3.73 Limited Slip differential in the rear.

IIRC my rear took just under 2 gallons (8 quarts) to fill from empty.

 

It is not nearly that much.

Assuming this F350 is a SRW model, you have a Ford Sterling 10.5" rear diff. Fluid capacity is 3.3L. You will need to buy 4 quarts and will have a bit left over.

As Brian said, it is limited slip. So make sure the oil has friction modifier built in. I'm pretty sure RP does, but check the bottle. Mobil 1 is also a good choice. Synthetic is required.

 

The best is to find a manual. I do have Dana80 what is the biggest (?) differential installed on those trucks and 2 gallon bottles had lot left in the second.
Always good idea is to measure drain oil for starting point.