2006 Pre Runner limited Slip Differential

Hi All, Probably one of the nicest features on this truck is the Limited Slip differencial, my previous 4WD trucks didn't have this feature, which required me to use FWD, now that I have a limited slip rear end, I don't need 4WD, even in snow. I don't do alot of off raod stuff so its all great, my question is, what can I do to preserve the differential and how long can I expect to make it last? I just turned 35K miles and found out Its time to change the Oil, I called the dealer, they recommend 80-90wt and they also add a special additive? does anyone know about this, can you buy it at any auto parts store?