A diesel fleet was powered with filtered, used frying oil (Anon, 1982). Used cooking oil and a blend of 95% used cooking oil and 5% diesel fuel were used. Blending or preheating was used as needed to compensate for cooler ambient temperatures. There were no coking and carbon build-up problems. The key was suggested to be filtering and the only problem reported was lubricating oil contamination (viscosity increase due to polymerization of polyunsaturated vegetable oils). The lubricating oil had to be changed every 4,000–4,500 miles.