The thicker the oil the more it lubricates that's a fact. The thinner oil is recommended primarily to save gas. Going up a little in weight doesn't hurt just don't go as far as 90 weight gear oil. You could run straight 30 weight oil and it wouldn't hurt. In Florida I run 5 or 10w20 in the summer and in winter I run 0 or 5w20. I could run 5w20 all year long with no detrimental results.
10w20, 5w20, 0w20 shouldn't make a difference in warm/hot weather. The second number (20) is the key consideration for hot environments. The first number is something to consider in cold/winter climates.
Personally, I'd run a 30 weight oil in a warm southern climate. I'm definitely switching back from 0w20 to a 30 weight oil in the very near future.
BTW, a blanket statement like the thicker the oil, the more it lubricates, is a bit of a reckless statement.
Today's engine tolerances are much tighter than in the past. At some point in the process, a thicker oil will actually lubricate less because the oil pump cannot pump with enough pressure to get the oil into the tight spaces between the rod bearings and crank journals, as well as the cam shaft bearings. The oil is simply too thick to be effectively pumped into those tight spaces, resulting in lower oil pressures and less lubrication. Thicker oil will also increase engine heat, as the engine and pump have to work harder to overcome poorer oil flow. There's a reason why newer engines don't recommend 20w50 oils anymore.
