The present invention relates to lubricating oils, and in particular to the use of lubricating oils with low sulphur content in combination with a low sulphur fuel to reduce particulate emissions of a diesel engine equipped with a particulate trap. Thus, there is provided the use of an engine lubricating oil having a low sulphur content in combination with a fuel having a low sulphur content, to reduce the emissions of nucleation mode particles from a diesel engine fitted with a particulate trap. There is also provided a method of reducing the number of nucleation mode particles in the emissions from a diesel engine fitted with a particulate trap, which method comprises using an engine lubricating oil having a low sulphur content in combination with a fuel having a low sulphur content.