Tribological and functional limitations of universal engine oils in hydraulic systems
Izr. prof. dr. Franc Majdič, univ. dipl. inž., Univerza v Ljubljani, Fakulteta za strojništvo
Ventil 32 (2026) 4 / Review articles / Technical article – 1.04
Abstract:
Universal motor oil can also function satisfactorily as hydraulic oil. However, if universal motor oil is used as hydraulic oil solely because of its high viscosity index (VI) – that is, because of its stable kinematic viscosity regardless of temperature changes – then this is not the right solution. This is, of course, related to the additives that increase the viscosity index. These additives are relatively large molecules that do not thicken the oil at low temperatures, but at higher temperatures form long molecular chains, which consequently increase the kinematic viscosity of the oil. The first viscosity index improvers, used in 1940, were made from melted rubber and were suitable for use in motor oil. Viscosity index improvers have been greatly refined over years of development. However, even today they remain sensitive to shear stresses during pumping. From the perspective of shear stresses, hydraulic fluids are subjected to the highest loads, which is why Group VI improvers are used.
Key words:
engine oil, hydraulic oil, additives, viscosity index
Copyright (c) 2026 Franc Majdič
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