All illegal Y/T items are removed from your player/house/vehicle every restart. Legal T/Y items remain in vehicles/player/buses. All 'I' items in vehicles are removed. Illegal items and any 'I' item can be stored in houses.
For saftey, sync data manually 10/5 mins before restart, everytime you sell/process a vehicle and/or buy gear. Also do this before logging/restart for precaution.