Grant, for summer usage either the 10W40 or 20W50 will be fine. Probably the greatest difference (if any noted at all) would be in engine & transmission noise. The base oil stock is a fair amount heavier on the 20W50 than the 10W40. Depending on the oil chosen you could also have a slightly better additive package on the 20W50 as that viscosity isn't to be used in emission compliant cars & trucks. Even the 10W40 isn't emission compliant from most oil vendors anymore so that could also contain better anti wear additives.Here in the maritimes our summers average about 20 to 25 degrees C. About 75 to 85 F. I tend to get stuck in traffic on the Bedford Hwy frequently, would there be any advantage to going to a thicker oil. I now use 10w40.