Following the classical Buckley-Leverett theory for the two-phase immiscible flows in porous media a non-linear evolution equation for the water-oil displacement front is formulated and studied numerically. The numerical simulations yield a physically plausible picture of the fingering instability known to develop in water-oil systems. A way to control the unrestricted growth of fingers is discussed. Distinctions and similarities with dynamically related Saffman-Taylor and Darrieus-Landau problems are outlined.