I looked in the service manual for my 2003 CTS. It has a 5L40-E transmission. Here's what it says:
Transmission type: 5 - 5 forward gears
Transmission type: L - longitude Mount
Transmission type: 40 - product series
Transmission type: E - electronic controls
Position quadrant - P,R,N,D,4,3,2
Case material - Die cast aluminum
Transmission weight dry - 161 lb
Max gross vehicle weight - 7,716 lb
This car has not only a engine oil life sensor, but also an ATF life sensor. After 9,000 mi the sensor still says, "100% life remaining." The transmission has no dipstick. It looks like changing the fluid is almost impossible for a do it yourselfer. The owner's manual says under normal service it is a lifetime fill. Under severe service it should be changed at 50,000 mi.