The direct breakdown voltage of degassed and oxygen-saturated oil has been measured as a function of gap length (28-200pm) using different combinations of etched aluminium, anodized aluminium and stainless steel electrodes. Polarity reversal tests revealed the presence of a crossover point in the breakdown characteristics analogous to that obtained for non-uniform field geometries. It is therefore inferred that the breakdown mechanism is initiated at local asperities on one of the electrode surfaces. The shapes of the characteristics were found to be either concave or convex with respect to the gap axis, depending on the material of the electrodes used. A concave shape could lead to a minimum in the average breakdown strength against gap length characteristic.