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.