Transmission fluid isn't going to hurt anything in a differential but you don't want to leave it in there if that's not what it came with. It has different lubricating properties. Similar gears are used in manual transmissions and many of those come with automatic transmission fluid instead of gear lube. I have an '80 Plymouth Volare that came with ATF in a manual transmission.
Front-wheel-drive cars share the same fluid between the automatic transmission and differential so that will tell you there's nothing to worry about with your car.
Monday, June 25th, 2012 AT 7:49 AM