Observation of quantum oscillations such as those in magnetoresistance and magnetization has long been an important experimental apparatus to study the Fermi surface in quasi-two dimensional organic conductors based upon a two-dimensional organic donor, bisethylenedithiotetrathiofulvalene. One of the most exciting features found is splitting of the oscillation peaks at the quantum number as large as 50, which has previously been observed only in the quantum limit. There have been two possible theories which explained qualitatively the observed splitting of oscillation peaks. The first and usually cited mechanism is the spin-splitting mechanism. The spin degeneracy is removed in the magnetic field and the Landau levels are splitted into two. The second and the more unfamiliar possibility is the splitting due to imperfectness of the two-dimensionality of our system. In this paper, we will focus on the second possibility where we can also derive almost all observed experimental data even though we could completely neglect the spin-splitting mechanism. We also compare the results with those derived from the first. Finally, we suggest some experiments which eventually clarify the question.