Add the necessary amount of Mobil 1 ATF fluid through the dipstick in the engine compartment. A normal 4L60E in a Z28 would take around 4 to 5 qts just for a normal changout of the fluid. It is important to remember here that much of the tranny fluid will stay in the torque converter and there is really no way to get it out when performing your own tranny fluid change so you'll be mixing your new ATF with the older ATF that was in there. This is OK and it is even OK to mix synthetic and normal ATF if you decide to switch to Mobil 1 synthetic ATF. In my case I am using a deep pan which takes about 2 qts more than a normal pan. However, I also have a Vigilante 2800 converter which is slightly smaller than stock so for me, the total I needed to add was 5.5 qts :)