I have a question on the effictivness of mixing oils to obtain a desired viscosity. I would like to use Mobil 1 synthetic oil in my car but Mobil only makes a 10w-30 and a 15w-50. I would like to get the desired weight of 15w-40. Is is it as simple as mixing equal amounts of the two weights to produce a 12.5w-40? Disadvantages?