What oil is supposed to be best for hot weather?

I switched from Mobil 1 10-30 to Castrol Syntech 10-40 and my engine feels like it runs quieter. In fact I can definitely tell a difference, and i get no clatter now since putting this oil in.

I'm not saying anything bad about Mobil 1 oil, it just seems like my engine likes to 10-40 a lot better. I'll try some Mobil 10-40 next time but I was in a hurry and the Syntech was the only 10-40 oil they had at Autozone at the time. A mechanic friend had suggested to me that I try that viscousity.

2002 Sportage 4dr 4x4 soon to undergo an Extreme Makeover!