I spoke to my mechanic about 40w oil. He said while it may not be a bad thing, the oil ports are small on these engines and may not be able to circulate the 40w viscosity as well as 30w, creating more friction. I used 5x-20 synthetic my last change. He was skeptical about that as well claiming while it is probably fine, during harsh conditions when the engine is at its max heat, it may be better to have the 30w viscosity.
So, who really knows what's best. I'm back to following the manufacture specs of synthetic blend or full synthetic. My book shows synthetic blend, but my dealership claims they now use full synthetic.