Insoluble contaminant or a varnish is a result of oil degradation by products and sometimes depleted additive molecules. This process is in most cases initiated by thermal stress placed on the oil. In tribology varnish becomes a significant problem for modern complex machinery lubrication systems as it has severe debilitating effects such as loss of operating clearances and heat transfer. Problems arising from varnish build-up in lubricants can be inhibited by timely oil analysis which gives us important information about oil-degradation level and current insoluble contaminants potential of oil. There are several laboratory testing methods to describe varnish potential. The most applied one is a colorimetric analysis, also known as MPC (Membrane Patch Colorimetry) as it tends to be reasonably quick and cost-effective. This study applies principles generally used for MPC test of turbine oils to measure lubricant generated insoluble contaminants of hydraulic fluids. For this purpose, in-service oil samples were taken from hydraulic circuits of rubber vulcanizing presses and analyzed using testing method based on method defined by standard ASTM D7843 – 21. Accuracy of this testing method is dependent on development time of tested sample. Therefore, we not only try to proof applicability of this method for hydraulic oils but also want to determine incubation time needed for efficient and accurate determination of concentration of insoluble contaminants in the sample.