Described herein are compositions and methods for using metal oxides to upgrade oil. Metal oxides may be used as catalysts to reduce the TAN of the oil by converting carboxylic acids such as naphthenic acids into non-corrosive products. In some cases, the conversion occurs by a decarboxylation of the carboxylic acid to produce CO.sub.2. A second process promoted by metal oxides is hydrocarbon cracking. Cracking decreases the viscosity and increases the API, and produces lower molecular-weight hydrocarbons that are useful for many fuels and lubricants. Reductions in TAN and the increases in API improve the quality of increase the value of oil.