We formulate a problem describing the onset of multi-diffusive convection in a horizontal porous layer. Our formulation collapses to a clear fluid case in a special limit. We study this problem, analytically and numerically, for the case of two diffusing components. We concentrate on the case when the boundary conditions at the two horizontal walls approach isoflux conditions and thus the critical wave number approaches zero. We investigate how the reduction in the critical wave number affects oscillatory instability.