To keep your new thermal compound fresh for future applications, always replace the cap on the syringe after each use.
The syringe should be stored tip down so that any separation between the particles and suspension fluid will be at the back end of the syringe.
Like any mix of particles that are many times heavier than the suspension fluid,
there may be some separation in the compound over time when stored in the original syringe.
Storing in a cool place like a refrigerator will minimize the possibility of separation over time. This does not affect the performance of the un-separated or remixed compound.
http://www.arcticsilver.com/pdf/appmeth/int/vl/intel_app_method_vertical_line_v1.1.pdf