For a car that is driven 80% or better on a highway a 7500 mile oil change interval is perfectly fine. I have a 300,000 Toyota 4x4 and had a 150,000mile 02 Elantra that prove it. PLUS the owners manual states 7,500 miles is ok for normal duty situations. As for 5W-30 getting better fuel mileage over 10W-30 it will be minimal at best because at normal engine temps the oil is at the 30wt level so there is no gain. Now a 5W-20 should get you alittle better but I am not sold on a 20W oil for full time use, my 07 Sonata has 5W-20 but I think I will be using a 5W-30 synth for the remainder of its life.
2007 Sonata SE V6 in Steel Gray w/Amsoil 0W-20 inside.
Independent Amsoil Dealer