2004 Infiniti G35 Slippage

Those arent the most reliable transmissions to begin with, so 100k isnt as bad as you may think. HAving the entire system flushed with Nissan fluid could help, but no guarantees. The incorrect fluid can cause damage to internal seals, clutch plates, basically any wearing item in the trans could be affected by the wrong fluid. All damage is repairable, you may be looking at a rebuild if it is slipping and has that many miles on it, I would stick with Nissan on this, have seen many trans problems from independent shops with Nissan products