2nd shift mushy at first

[chevyrulespops]

upon inital start up and when its cold if I were to run the car right away I would get a delayed 2nd shift...and I have to be careful not to rev up the engine high or a noticeable thump occurs...i was thinking of putting in an aftermarket oil additive like Lucas to try to help this erratic shifting.. When the car is fully warmed up which could take up to 30 minutes of driving or more, then the shifting becomes normal..anyone else experience this? and what has helped for you? Im using the atf #4 I believe