Hmm tricky one this, the carrier may require some sort of proof the engine has been drained, ie statement that engine has been "Purged", also an idea to add to the description of goods on the airwaybill, "not restricted" as per 8.2.6 of the IATA dangerous goods regulations book. If the purged status cannot be proved, (as pointed out in an earlier posting), it falls under UN3166, Class 9, "Engines internal combustion gas/liquid powered", I am afraid to say.
Cheers, and Best of luck.