The EM-Drive, as long as it is considered a closed system, explicitly violates the conservation of momentum and Newton's well-known third law: however, it would appear, according to several tests to date, that the device may concretely deliver a certain thrust without a detectable exhaust. The question is: can the EM-Drive be actually regarded as a closed system? We have elsewhere tried to provide a qualitative answer by resorting to a theory based, amongst other hypotheses, upon the existence of a further spatial (hidden) dimension. In this paper, the whole revised theory is step-by-step expounded, avoiding, for the sake of brevity, some aspects that, notwithstanding their undeniable relevance, do not concretely contribute to the achievement of our main goal. We consider a Universe belonging to the so-called oscillatory class. Firstly, we formally show that, as it is well known, a simple-harmonically oscillating Universe is fully compatible with General Relativity. Then, we carry out an alternative deduction of the mass-energy equivalence formula as well as of the Friedmann-Lemaître equations. Finally, by resorting to an opportune writing of the conservation of energy (carried out by taking into account the alleged extra spatial dimension), we implicitly obtain a new definition of closed system, so providing an answer to the question previously posed.