well im thinking about draining and putting in new gear oil in the rear axle, cause this truck has 233,000 miles on it and i know the po has never changed it or refilled it. i went last knight and picked up 4 bottles of valvoline 80w-90 and the guy asked if i need the additive that goes in the rearend. i answered i dont know because i thought the additive was for limited slip, and i dont know if its limited or posi. how can i tell if its either one and if i need the additive?