It is preferable to use the manufacturer recommended viscosity for the heat transfer unit. To thick and the oil will not flow properly reducing the efficiency of the heat transfer unit possibly causing a overheat condition. To thin a oil can also adversely effect the efficiency of the unit.
Also mineral heat transfer oils are generally not blended,i.e. straight from the refinery ensuring that there is no blending of light and heavy oils to achieve the viscosity(reduce volatility and flash point issues)