Spent lube oil (SLO) was mixed with kerosene or diesel in various percents and then burned in a laboratory combustion unit. The effects of air to fuel ratio (A/F) on the flame temperature, combustion efficiency, thermal efficiency of the unit, and combustion product gases were studied. Results have shown that the maximum temperature of the flame was obtained at the center axis of the flame, and the flame temperature was decreased as A/F ratio or % SLO in the fuel mixtures was decreased. The maximum percentages of SLO that can be mixed with kerosene or diesel and then burned in the combustion unit are 40% and 15%, respectively. Combustion and thermal efficiencies were increased as A/F ratio was increased and then decreased for higher A/F ratios.