for 03 tacoma 4x4 trd. not my truck but im going to do a rear diff oil change on it. it has i believe 160k miles and never had one done, so now do i use synthetic or regular? how do i know if this truck have lsd? theres a switch that says r/r locker, so im assuming it has a locker. my bros truck and since im doing my tundra, im going to do his. i got some regular 75-90 valvaline gear oil, hopefully that will work. how many qts to use?

I used royal purple 75-90 , and you dont have to add the limited slip additive , it already has in it . Mines took 2 1/2 qts

Look on the diff. There should be a sticker that says, LSD only. Also if the truck came with a factory tow package it most likely has the limited slip.