Method and control device for determining a viscosity parameter of an oil

The method involves performing hydraulic control of a gas shuttle valve (2) and adjusting a hydraulic component i.e. magnetic valve (8), from first position to second position with a control signal. Time duration required for changing the component from first to second positions is consulted as measure for viscosity characteristic. The viscosity characteristic is evaluated in combination of determination of a measure for oil quality, recognition of undue engine oil, determination of an oil change interval, diagnosis of the hydraulic component and a valve controller. An independent claim is also included for a control device for determining viscosity characteristic of hydraulic fluid.