Methods, media, and devices for determining the deterioration of an oil by determining the viscosity of the oil from fluorescence rise-time are provided. A sample of an oil from a machine may be obtained. The oil sample may be irradiated using a light source such a laser to induce fluorescence in the oil sample. The induced fluorescence may be filtered and collimated before being detecting by a photodiode. The relative intensity of the fluorescence may be measured over a time period at a sample rate and used to determine the fluorescence rise-time of the oil sample. The viscosity of the oil sample may be determined from the fluorescence rise-time. The determined viscosity may be compared to viscosity threshold value to determine the deterioration of the oil.