If you're not tracking, you'll be fine with the stock oil even in higher ambient temps. Without actual empirical data on oil pressures+temperatures, there's really no point in just guessing on diff oil weights for the hell of it, as is the pastime of so many forum dwellers. Your car will run just fine on the factory oil, it was tested in your ambient temps as well as absurdly cold climates as well.
Just installed the mishimoto catch can kit today and decided to change the oil and oil filter whilst we were at it. When i inquired about a oil filter via the toyota dealer where i bought the car they said they had none !
A quick search found a toyota dealer with 2 in stock but the dealer was a 1hr drive from where i live so the parts department suggested ringing BMW. low and behold they have stock of a z4m40i filter. Toyota charges $24 for the oil filter. BMW $35 for essentially the same thing . Toyota oil charges $126 for a 5L bottle incidentally and you need 6.5L for an oil change.
Whilst i was at BMW i inquired what they used for oil and he replied that they used castrol edge 0w-40 . I ended up using Penrite enviro+ 5w-40 oil