Im needing to know what kind of diff i have and if i have to use a additive with the gear oil.

It came out of a 1989 s5 NA coupe 5 speed that had PW, 4 PISTON BRAKE, SPOILER, SUNROOF, REAR WIPER, AND AUTO SEAT BELTS if this might help.
Im thinking a gxl???
not sure the car was painted so theres no badge on the fenders.

I have the diff pulled out and when i turn the drive shaft input flang on the diff the two output flanges for the half shafts spin at the same time in the same direction.

so would this be a type of LSD?
and if so do i need a certain type of additive that needs to be added?

 

89 - 91 used vlsd...even if your car came lsd no additive is necessary.

 

S5 lsd diff does not need any additives.
Viscous lsd, its self contained. Just add gear oil.