Experimental design methods have been successfully applied to develop and optimize the process of synthesis of butyl esters from rapeseed oil (RSO), the most common oil feedstock for biodiesel production in Europe and used frying oil (UFO), as a cheaper raw material, using biobutanol, and potassium methoxide (KOCH3) as catalyst. The optimum conditions were found to be a catalyst concentration of 1.1% and 0.9%, an operation temperature of 78 degrees C and 80 degrees C for rapeseed oil butyl esters (RSOBE) and used frying oil butyl esters (UFOBE), respectively, obtaining ester yields of 96.86% and 96.54% with 6:1 biobutanol/oil molar ratio. Results show that biodiesel produced using biobutanol as alcohol in the transesterification process improved cold flow properties in terms of cloud point (CP), pour point (PP) and cold filter plugging point (CFPP) without significantly affecting the other fuel properties. In order to determine the effects of long storage on oxidation stability, the biodiesel samples were stored for a period of 12 months, the analysis of fuel properties: peroxide value (PV), acid value (AV), iodine value (IV) and viscosity (v) have been applied in oxidation studies. According to the results obtained, the RSOBE and UFOBE samples showed a good oxidative stability during the storage period. The resulting butyl esters can be used as a diesel fuel substitute, since it matches the European Biodiesel Standard EN 14214. (C) 2013 Elsevier B.V. All rights reserved.