My understanding is that mid-grades are a mixture of regular and supreme and thus mid-range also would have the detergent that you say only supreme has
When I go to Shell stations or any station for that matter I usually get the middle grade presumably is a mixture of 87 and 93 (91) so I assume it has a hefty dose of detergents in it
I've had very bad experiences with Mavis tire in New York City; I once went in for tires and they told me that I needed new struts.. The struts only had 11,000 mi on them after that I never went back to Mavis
I avoid Mavis tire at all. Costs; I went there about 4 years ago for a couple of tires and they said I needed new struts... The struts were only 4 months old