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Cavitation can cause severe damage in journal bearings as a result of the collapse of cavities in the vicinity of the bearing wall. It has been postulated by others that this phenomenon may be mitigated by the presence of polymeric additives in the lubricant. The primary purpose of the additives is to reduce the dependence of lubricant viscosity on temperature, but polymer addition also renders the oil mildly viscoclastic and slightly shear thinning. The objective of this work was to explain how lubricant shear thinning might reduce vapor cavitation. The mechanisms by which cavity formation occurs in dynamically loaded journal bearings were investigated. The pressure distribution in the complete film was computed as different time instants, and this allowed one to follow the periodic growth and collapse of bubbles. Results revealed that cavitation arose due to two separate mechanisms.