The limitation of fuel entry into the oil sump of an internal combustion engine during operation is important to preserve the tribological properties of the lubricant and limit component wear. For observation and quantification of the effects leading to fuel entry, a highly sensitive and versatile measurement system is essential. While oil sampling from the sump followed by laboratory analysis is a common procedure, there is no system for automatic sampling of all the positions and fast online analysis of the samples.For the research project ‘Fuel in Oil’, a measurement system was developed especially for the described tasks. The system is placed next to the engine in the test cell. Sampling points are the target point of the fuel injector jet and the liner below, the oil sump and the crankcase ventilation system. The system consists of a microliter volume and an aerosol sampling setup, a probe evaporator, an isothermal gas chromatograph and a triple quadrupole mass spectrometer with a modified ion source.To quantify the fuel emission from the cylinder wall into the exhaust, an online exhaust gas measurement was carried out using a direct inlet system with the same mass spectrometer.With the use of the described setup, it was possible to observe and quantify the effects leading to fuel entry into the oil sump as well as fuel leaving the sump.