the sae 50 will be heavier than 15w-50 even at high temps. the additives used to make it a 15w when cold will hurt it's hi temp capabilities after say 1,500 of use. sae 50 is basically for racing or real heavy duty applications where cold stats are not an issue. like already mentioned your mechanic is tard and the heaviest you should go is a 10w-40, but even still if it gets below 20 degrees you need a 5w-30 or 5w-40 if that is made.