Aiming for cleaner and more efficient energy from the internal combustion engines makes necessary to ensure the special conditions for exploitation of alternative fuels. The engine vibrations are primarily understood as effects of mechanical failures, but they are also a subject of the fuel combustion effects. These effects depend on the fuel type and its ability to complete the combustion process. The vibrations of a diesel engine were measured and analyzed with a frequency spectrum calculated with fast Fourier transforms. The engine was operated with a fuel blend of 10 % recycled lubricating oil with 90% diesel fuel as well as with neat diesel. It was found that the engine operation with this fuel blend has a lower vibration level in comparison with the use of neat diesel fuel. The goal of this research is to determine the properties of the fuel blend, which provide more stability to the engine by means of vibrations reduction. The fuel properties that influence the engine stability are the flash point, the cetane number and the heat value. Improvements of these properties allow keeping of the engine performance with better stability during its operation at different speeds. Testing of the engine performance by means of vibrations analysis is a powerful tool to determine engine behavior from the functional and endurance point of view. Vibrations measurements of the diesel run on different fuel types and properties make possible to choose and determine the fuel properties that bring higher stability to the engine.