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In this paper, we report on a portable, compact and inexpensive fluorescence sensor for monitoring the ageing of retail lubricant oils as a result of the operation in an internal combustion engine. The proposed sensor consists of a UV laser diode, an optical fibre probe for simultaneous excitation and collection of the fluorescence and a USB powered palm-size spectrometer connected to a laptop. The sensor was applied on a set of oil samples, taken from production engines by a leading car manufacturing company, at various mileages. The subtle variations in the spectra, caused by the ageing process of the oil, are resolved using two methods: principle component analysis (PCA) and artificial neural networks (ANN). Their performance is compared with regard to utilization in a portable dedicated system for continuous in-situ monitoring. Because of the "fingerprinting" approach, which avoids detailed interpretation, the sensor has a wide range of application areas.