I'm using garmin 96, g7towin for track download, and SeeYou software.
Do I have to split tracklog of several consecutive flights if I want to
upload only the best one to the OLC, or I can upload the whole tracklog?!
What will happen then?
Well, garmin devices have a strange feature - if you SAVE a tracklog after
each flight, you lose all altitude data. You have to download it as an
active log to preserve all data.

When using g7towin, I tried to separate the flights using "start new
segment" option and saving it as .g7t or.plt file (no option to save it as
..igc first) and then converting them to .igc file (save as .igc file
option), but then I lose securitiy G line at the end of each track.

Well, do you have any idea how to that?