In the case of some engine oils, under specific conditions, engine oil additives may precipitate in the form of needle crystals which cause clogging of oil filters. A simple laboratory test has been developed to study the formation of needle crystal precipitates in engine oils. The influences of various conditions and the combination of engine oil additives on this formation were investigated using this test procedure. This laboratory testing showed that needle crystals form when the engine oil containing a magnesium-based detergent comes into contact with both water and carbonic acid gas under restricted conditions. It was discovered that there is a large difference in the formation of needle crystals between different magnesium-based detergents. The addition of suitable additives could prevent the formation of needle crystals even in those oils which cotain magnesium type detergents which are prone to form needle crystals.