IMHO outside temperature may come into play only at below freezing points during cold start and there too once it attain optimum temp then it should be running fine. In summers the cold start issue is also not there then y this debate of thick and thin oil. we need to clear our concepts. Lets tag @xulfiqar @2c and @capsat to guide us.
Personally I have clocked 50k kms on my corolla using 20w50 Toyota motor oil. All is well ma sha Allah. Initially when i got that car was in a fix about this thin oil n thick oil n esp in winters. I was guided by @2c and gave examples of ppl running 20w50 in below freezing temp of canada. The other reason I am sticking to toyota oil from dealership is at least it is not fake, as market is flooded with fake oils.
Still we need a good tutorial discussion on this to clear our concepts and misconceptions.