Why has my engine oil turned gray?

If it is a whitish-grey colour then possibly you have blown the head gasket of your engine....expensive job to fix. Basically it can "blow" by overheating - which means the gasket get ruined...fails to do its job and the radiator water flows through the engine sump rather than "around it"...hence water and oil mix...equals grey or white. Your mechanic can confirm whether or not this is the cause.