SR5,
I have never seen in any owners manual, a listing of the correct lubricant weight oil for the engine in question. When it is stated 5w-20 preferred, I automatically assume that a thicker oil like 0w-40 or even 15w40 will outperform what is the "preferred" oil. I'm totally basing this on personal experience and in no was base my decision on factual data.
Nuff said,
xtell