A.N. Farhanah and S. Syahrullail, “Tribological Behaviour of Zinc Dialkyl-Dithiophosphate (ZDDP) as a Lubricant Additive in RBD Palm Stearin”, J. Adv. Res. Fluid Mech. Therm. Sc., vol. 11, no. 1, pp. 19–26, Jul. 2015.