People who work on them distinguish between the electrically powered unit called the “motor” and the engine which it starts; but even in auto-parts stores the stuff which by that logic should be called “engine oil” is marketed as “motor oil.” Similarly, the English go motoring on motorways. In everyday American discourse, the terms are often interchangeable (you can buy a powerful engine for your motorboat), but you may embarrass yourself if you don’t make the distinction when talking to your mechanic.