Oil mists, which are colloids composed of solid core particles surrounded by oil, are commonly removed from air by fibrous filters. However, unlike the filtration of pure solid or pure liquid particles, mist filtration is not yet fully understood. This study performed a series of experiments to investigate filter performance throughout a clogging process and evaluated the influence of oil content on the variance of pressure drop and filter efficiency. A model was developed for predicting the efficiency of the colloid particles and used to explain how the variance of the oil content changed the efficiency. Results showed that the filter became saturated, showing a constant pressure drop and efficiency, after a relatively short period of fluctuation. The particles with higher oil content used less time to reach the equilibrium state, and had a lower pressure drop and higher efficiency. The amounts of oil coated on the solid cores influenced both the particle diameters and the saturation of filters. Filter efficiency can notably increase with decreasing saturation until saturation was less than 0.5, and the efficiency of particles that were most affected were those with diameters of approximately 30 nm and 500 nm.