15W-40 should be fine in Hotlanta.

The viscosity scale is not linear. For example it sounds like a 15W-40 oil is 50% thicker when cold than a 10W-40 oil but it really is not.

According to API,

a 5W-xx oil should have a maximum low temperature cranking viscosity of 6600 cP at -30 deg C.

a 10W-xx oil should have a maximum low temperature cranking viscosity of 7000 cP at -25 deg C.

a 15W-xx oil should have a maximum low temperature cranking viscosity of 7000 cP at -20 deg C.

Since it is not linear, it is hard to compare on a linear scale (the scale we are used to). Some oil experts said, if forced to say it, that a 15W-xx oil is really like a 12W-xx oil if it is compared linearly.