A colour-indicator titration, for example using test method ASTM D974, can be carried out to indicate relative changes that occur in an oil sample during its use under oxidising conditions. A sample is dissolved in a solvent mixture of Toluene/ Propan-2-ol with 0.5% deionised water.  A methyl orange indicator is added and the solution is titrated using alcoholic potassium hydroxide. The end point is indicated by a colour change from orange to green.