**6. Stability**

All petroleum products will degrade when exposed to certain environmental conditions such as high temperatures, oxygen, mechanical shear, and UV exposure. This degradation will often cause plugging of filters and engine deposits. Instability involves the chemical conversion of precursors to species of higher molecular weight with limited fuel solubility and tend to be nitrogen- and sulfur containing compounds, organic acids, and reactive olefins. The number and rate of these deleterious reactions depends on the concentration of reaction precursors, the concentration of oxygen, catalytic species presence (such as metals), the light intensity and storage temperature.

Oxidation stability is a measure of the fuel's resistance to degradation by oxidation. In the oxidation stability test, the automotive fuel is subjected to conditions which promote oxidation (by subjecting the fuel to oxygen and an elevated temperature for a fixed time). The pressure is monitored for gasoline by ASTM D525 (or equivalent ISO7536 [3] and IP40 [4]) as pressure drops when smaller gasoline molecules oxidize to form larger and less volatile ones. As gas oils are not so volatile, the product after oxidation is cooled and the total insolubles are measured by ASTM D2274, D4625, D5304 and UOP413. These methods differ in oxygen administration technique (continuous bubbling or by pressure vessel), oxidation time and testing temperature. ASTM D2274, D5304 and UOP413 are rapid screening tests used for comparative purposes for evaluation of the tendency to form sediment in storage. In order to accelerate the oxidation process the testing temperatures used are 95°C, 90°C and 100°C respectively. ASTM D2274 tends to be mostly used in current
