Castrol synthetic or Valvoline?

It is odd. But I go from 23 mpg to about 20 sometimes less depending. Almost like the oil has broken down to much and is causing to little resistance or more. Idk it's definitely odd. Running synthetic I should be able to go 5-6k before changing oil and filter. That's why I'm trying the extended stuff. Claims 15k protection. We will see. When I was using normal oil I'd hange it 3k and never noticed a change unless I went about 1-1.5 k over.