zic atf is a dexron 3 clone - which is most probably the wrong ATF for your car. I bet it requires a highly friction modified ATF.
another point - if your ATF change has resulted in a transmission shift that cannot even be felt at higher rpm shift then your clutches are slipping excessively which will break down the ATF and hence the entire transmission very soon.
In any automatic shifting gearbox you should be able to feel a shift happen at high rpm and load (even in lexus) if you dont then your transmission is setup wrong (oldschool) or you have the wrong ATF (newschool)