Rule of thumb cant be applied to the oil feeding in to engines. Kixx Pao is fed to newer engines that are previously running on lubricant viscosity nearest to 5w-40. However, if a car is previously running on 20w-50 viscosity oil and is fed with 5w-40pao or fully synthetic 5w-40 viscosity oil then it is sure to cause oil and engine performance depletion. Btw we too have multiple oil brand distribution and we are in it for last 4 generations.