if I remove the engine and gearbox from my 89 5.3, then separate them. will I at any point lose all the tasmission oil? many thanks lads

 

No, unless you remove the sump and drain the oil first, which is recommended!
Greg

 

don't forget the torque converter holds a lot of transmission oil as well,

use of the recommended bracket is advised to stop it falling out the gearbox during removal if separated from the flex plate ( fly wheel ) during removal

BB