Why is your oil in your car watery?

If you mean thin, oil breaks down with heat and wear. It needs to be changed every 2-3 thousand miles. It can also be from fuel getting into the crankcase. Either from a bad fuel pump if it has a mechanical pump on the engine or from too much fuel being put into the engine either by a bad carburetor or injectors.

If it is milky that is water in the oil probably from a head gasket.