One of the important factors when selecting the more appropriate hydraulic fluid for use in applications is definitely its long service-life. By using hydraulic fluid with long service-life, together with enhanced condition monitoring of the fluid, we can significantly reduce costs and minimise environmental impact due to less lubricant production, logistics, and disposal. This paper describes a novel method for testing the durabilities of different hydraulic oils, followed later by addressing the condition-monitoring of hydraulic oils throughout their life-cycles. Particular emphasis is placed on quantitatively assessing, in detail, one mineral oil's condition and its remaining useful lifetime. This paper proposes a hybrid mathematical model based on gathered data from previously conducted oil-ageing tests. The presented model is able to successfully and accurately assess the oil's condition and its remaining useful lifetime after the oil has only reached one third of its lifetime. (C) 2014 Elsevier Ltd. All rights reserved.