I switched from Amsoil 5W30 to Amsoil 0W30 last month, and noticed a small bump up in mileage on freeway slogs. Mind you, the freeways here are limited to 62mph, or 100km/h and I generally just set the cruise at 65mph for best mileage, lowest noise, and ease of having everyone go around me with my cruise control on - makes for the most relaxing drive.
I can't really say I've noticed an increase with driving around town, but don't forget when calculating in mileage that adding even one 200lb passenger can have an effect on mileage when you calculate. There are tons of factors involved that you have to take into consideration. With 5 ppl in my G6 last week (tight squeeze it was), and some things in the trunk I still managed 23mpg with 80 miles highway and about 40 city. I consider anything over 20mpg with a V6 carrying 1000lbs of lard arses around a good show!
'11 Ram SXT 4x4 quad cab - 5.7L V8; 128,200kms
'09 Malibu 2Lt 3.6L - 140,000kms purchased 1 March 2019
'06 Pontiac G6 GT - 3.5L V6; 162,000kms (rear ended 15 Feb 2019, and given totalled title by insurance)