For testing Bio-Diesel storage stability there is no satisfactory method available at this time. Also storage stability of mixtures of Petro-Diesel and Bio-Diesel is unpredictable and does not have an established test method. PetroOXY is projected to establish a new method for evaluation of the storage stability of liquid fuels and an automatic instrument to be made available to interested parties in the industry, in order to give good repeatable results in a minimum of time with best possible user safety. The method and the instrument are designed in a way not to exceed the average testing time of other test methods specified for releasing a fuel from any production facility or other storage. The defined event is Oxystab = Elapsed Time till (Max-Pressure - 10 %). The PetroOXY-Instrument is a test chamber receiving a volume of 5 ml of sample, with a micro-processor controller, an automatic oxygen filling and relief device, an automatic heating and re-cooling device, an automatic pressure detection system and an interface to transfer test data to a PC and a display to show the test result independently. The instrument is only 28 cm wide and up to four of these can be grouped in one single conventional fume-hood. The productivity is up to ten times better than ASTM D 525 and between two and five times better than the Rancimat. The instrument only needs supply of electric energy and Oxygen for filling and compressed air for cleaning.