Gas entrainment in engine oil circuit generates many problems. This is especially true for hydraulic tappets. A new oil gas content measurement apparatus, the SMAC, has been developed by IFP and D2T. It is able to measure the volumetric gas to oil ratio at any point of the engine oil circuit while running, without influencing the pressure or temperature of the oil. Some engine tests have been conducted with PSA. First results show that a steady gas content is achieved only slowly. Gas content is very sensitive to engine speed and oil temperature (i.e. running point effect). Gas content is very different from one location to another in the oil circuit (i.e. architecture effect). Oil formulation also influences gas content (i.e. oil effect) and the SMAC method does not rank the oils in the same way as the ASTM methods.