This part of ISO 4263 specifies a method for the determination of the ageing behaviour of gear oils of categories CKC, CKD, CKS and CKT as defined in ISO 6743-6[5]. The ageing is accelerated by the presence of air and elevated temperatures, and the degradation of the oil is assessed by the change in kinematic viscosity at 100 °C, the insoluble content (precipitation number), and possibly the acid number increase, the sediment content by filtration, the additives content variation and the infra-red oxidation.