New trends in the development of combustion engines lead to changed engine operating conditions which also result in changed loads of the engine parts and functional components. The application of regenerative fuels and new exhaust after- treatment systems can for example lead to an increased input of fuel into the engine oil. Oil dilution is an issue that impacts the engine operation and life cycle and therefore gains importance during the development process of combustion engines. In order to improve oil dilution to preclude destructive amounts of fuel in the oil it is necessary to investigate and optimize the fuel in oil sorption and desorption processes.At the University of Applied Sciences Regensburg a new measurement technique has been developed that realizes the online analysis of the fuel quantity in the engine oil. The measurement technique enables a total as well as a selective detection of fuel hydrocarbons so that besides the detection of the total fuel amount in the oil it is also possible to study the sorption and desorption behavior of single fuel components into and out of the oil.Investigations were carried out with the measurement technique on gasoline and diesel engines at engine operating conditions with high oil dilution risk. The main focuses were for gasoline engines on the study of cold start and warm up behavior of the engine under cold ambient conditions with fuels containing varying amounts of ethanol and for diesel engines on the study of late injection strategies for particulate filter regeneration.