here are many contaminants that could lead to the alteration of oil viscosity. Diesel fuel
is one of the deteriorating contaminants for diesel engine oils. It is among the main factors that give
high impact towards the deterioration of engine oils as one study has found that, in average, 0.36% of
total fuel consumption ends up in the crankcase. Many studies have demonstrated the effect of fuel
dilution on the wear characteristic. However, there are limited studies that focused on the percentage
amount affecting the wear characteristics together with other influencing factors such as load, speed
and temperature. In this research, used motor oil - Gibson 20W50 had been mixed well with fuel
percentages of 0, 1, 3, 5 and 7, hence creating 5 samples that were run under different speeds. The
experiment is conducted by following the ASTM 4172 standard. Based on the results, the fuel
contamination in used motor oil was considered limiting at 5% contamination and became totally
deteriorated at 7%. Through a Stribeck Curve, it could be understood that wear is inversely
proportional to sliding velocity, where it is one of the lubrication parameters together with speed and
load. Rough surface on the balls indicated that abrasive wear has occurred due to the use of used
motor oil