An experimental study on co-firing used engine lubrication oil (UELBO) with a gaseous fuel such as LPG in a small furnace is conducted. A wide range of fuel and air admission conditions is considered. It is found that when even a small amount of UELBO is co-fired with the gaseous fuel it significantly enhances thermal radiation capabilities of the gaseous fuel flame. For example, radiation capability of LPG flames can be enhanced by about 80% when UELBO at a rate of about 0.5 g/s is co-fired with 1 g/s LPG. UELBO-based results are also compared with ones obtained when a diesel fuel is co-fired with LPG in the same furnace, under operating conditions similar to those of UELBO/LPG tests. The comparisons confirm once more the significant role played by UELBO in enhancing radiation heat transfer from gaseous fuels flames. They also highlight UELBO as a renewable energy source with an attractive potential. (C) 2008 Elsevier Ltd. All rights reserved.