This study is an experimental work evaluating the thermal performance of dual fuel furnaces fueled with blends of diesel and used -engine -lubrication oil (ULO), co-fired with LPG (Liquefied Petroleum Gas). ULO is regarded here to be a renewable energy source. The word "renewable" is used in the present context because waste engine lube oil will be present and will last as long as there are engines in use. The paper addresses enhancing radiation from gaseous fuel combustion such as LPG, by co-firing it with some small amounts of a lower grade liquid fuel with higher radiation propensity. The liquid fuel used is a mix of ULO and diesel. Elemental measurements conducted revealed high metals content in the used ULO, as compared with the metal content in unused fresh oil. This is believed to be the reason behind the better radiation capabilities of the flame observed when ULO is used. Results show that substituting 30% of LPG mass by a liquid fuel mixture composed (in this study) of 20% ULO and 80% diesel increases radiation to furnace walls by about 1% above the level achieved when only LPG is used with the same total equivalent amount as that of the liquid mix plus the LPG portion. 20% ULO in the liquid fuel mixture corresponds to about 6% of the total fuel mass supplied to the furnace. To highlight the role that might have been played by the excessive amounts of metals in ULO regarding radiation, runs under the same conditions have been repeated, but with fresh oil replacing ULO in the liquid mixture. The results confirmed once more the superiority of ULO when it comes to radiation enhancement. The study summarizes also some results from a previous research, conducted by the first author of the present work and co-authors, assessing used engine lube oil against waste liquid oil with biological origin namely a "used cooking oil". This comparison further supports the use of ULO as a fuel supplement that aids heat transfer by radiation in furnaces. In addition, the study includes some measurements characterizing ULO such as viscosity at different temperatures and the calorific value. This study is believed to give useful insights for design, operation, and control in applications such as radiative boilers. (C) 2012 Elsevier Ltd. All rights reserved.