It is currently assumed that churning losses can be described by using only two physical parameters representative of the lubricant properties, i.e., density and viscosity. To verify this hypothesis, a number of transient measurements were carried out on a specific gear test rig over a range of oil temperatures. It appears that, for high temperatures and/or high rotational speeds, the drag torque can suddenly increase with an increasing Reynolds number. Extensive on-line lubricant aeration measurements were conducted showing that this particular behaviour can be related to churning losses when the fraction of air in the lubricant reaches a certain threshold. In order to quantify the influence of oil sump aeration on churning losses, a simplified original model, based on surface tension and lubricant aeration is proposed. This study shows clearly that density and viscosity are not sufficient to estimate churning losses under some specific operating conditions and emphasises the need to account for other physical properties of the lubricant.