Am I correct in assuming that a 5W/30 wt. oil is the same as a 10w/30 when used during the summer? Aren't both a 30 wt. during summer temps? Isn't the difference during the winter when temps are lower and the 5W/30 flows easier?
0W30, 5W30, and 10W30 will all flow the same (to the extent allowed within the range of "30" viscosity) when hot. The difference is in the cold flow.
Note that by "cold" I mean startup temperature, and "hot" is operating engine temperature (~100 C). 5W30 will flow better cold than 10W30 all year, not just in the summer, but it would be more noticeable in the winter. Even in the summer, your engine is far below 100C at startup.
Some brands will use more of a a better base oil in the 5w30 to compensate for the thinner base oil. For example, I recall Durablend had a lot more Group 1 base oil in the 10w30 than in the 5w30 according to the MSDS's.