Thruster cartridge seals are filled with oil.When prepared for storage or shipment at the factory, the central core where the shaft goes is filled with silicone gel to keep the oil from leaking out. Cartridges should be stored in a cool environment so the gel does not liquefy and leak out allowing to oil to leak out as well. It is also important to keep the cartridges in clean environment so that the do not pick up dirt that can lead to shaft scoring or premature failure of the seals.
Storing cartridges in a low pressure environment can also cause leakage due to internally higher differential pressure. When transporting cartridges by air, it might be better to pack them in carry on baggage instead of checked baggage.