I ride a 2015 Yamaha FZ-07 (around San Francisco / Bay Area), which is water-cooled and supports 10w30, 10w40, 10w50, and 20w50 motorcycle oils.
However, the manual says not use 10w30 oil when the ambient temperature exceeds 70F/21C (and recommends 10w40 or heavier instead).
Why are lighter oils contraindicated for hotter ambient temperatures?
If I use a full synthetic oil (which is probably going to be hardier), is it still bad to run a lighter oil when it's hotter outside?
The engine temperature itself is much hotter anyway, and stays pretty much the same (180F/82C) no matter what the ambient temperature is.