Mine was a complete overhaul. Still far short of 12.8 quarts.
In quarts it is 13.5...
(12.8 figure is litres).
Unless there's 2.3 litres inside the box internals then either the GM 7.3 litres figure is wrong or my dipstick is the wrong one. Should we all measure our dipsticks and report back?
Haynes says 7.7 Qrts for drain and filter change - taken straight from GM literature presumably.