The earliest gas turbine engines were developed using straight mineral oils but the operational requirements for low temperature starting either on the ground or at high altitude (re-lights) led to the development of a range of straight mineral oils with viscosities far lower than those of conventional aircraft engine oil of that time. For example oils with viscosities between 2 mm2 /s and 9 mm2 /s at 100C became standard for gas turbine engines compared with viscosities of 20 mm2 /s to 25 mm2 /s at 100C for piston engine.