I have a 2003 G35 auto, and I'm about to change the tranny fluid for the first time in a long time. I'm going to follow the service manual and replace the fluid by running out the old fluid from the radiator cooler hose return side while filling from the charging pipe.

How many quarts of fluid should I buy to be sure that I have enough to complete this job? The manual says fresh fluid should be a 30-50% increase of the stipulated amount, but I don't know how much that will be because the fluid hasn't been changed for a long time, so I'm sure I'll be "flushing out" quite a bit of dirty oil.

 

Transmission fluid in an AT is 10 3/4 quarts. If you are going to flush it completely I'd use 15 quarts total.