Synchronization phenomena, frequency shift and phase noise are often limiting key factors in the performances of oscillators. The perturbation projection method allows to characterize how the oscillator's output is modified by these disturbances. In this brief we discuss the appropriate decomposition of perturbations for synchronization and phase noise analysis of planar nonlinear oscillators. We derive analytical formulas for the vectors spanning the directions along which the perturbations have to be projected. We also discuss the implications of this decomposition in control theory and to what extent a simple orthogonal projection is correct.