to warm up the ATF - take the car on a high speed drive for 25 minutes. Then check the level. - There is no issue with anything like going uphill or downhill - the transmission can take it.
the usual failure points on this transmission are the accumulator springs breaking (changed from the lower sump) - governer hanging from dirt/debris - clean it out - its installed in the tail housing of the transmission, vacuum modulator leaking - it will cause bumpy shift or sometimes very harsh shift, this part has been updated by GM.