It all depends on if the tank was bone dry of 93 when poppers
put the 87 in it and how much of the 87 he put in.
Its a ratio between the existing 93 and the amount added of
the 87 octane fuel and how much he hot rodded it or if it pinged
while he pulled the steepest hill under load.
Hide the fricken keys or explain the proper refueling procedure
to whomever you entrust your key to. Unless he bought it for
you then you are under his thumb and there is nothing you
can do about it....

Seems like you need to add "The Club" to your list of mods....
p.s. the 'made with JET fuel' octane booster is no good
for your fuel system Silly octane booster makers, dont you
know 'JET FUEL" is mostly just a hyped up super clean version
of Kerosene? You wanna put kerosene in your stang and then
advance the timing for 93 octane?
Kerosene, sometimes spelled kerosine in scientific and industrial usage,[1] also known as paraffin, is a combustible hydrocarbon liquid. The name is derived from Greek keros (κηρός wax). The word Kerosene was registered as a trademark by Abraham Gesner in 1854 and for several years only the North American Gas Light Company and the Downer Company (to which Gesner had granted the right) were allowed to call their lamp oil kerosene.[2] It eventually became a genericized trademark.
It is usually called paraffin (sometimes paraffin oil) in the UK, South East Asia and South Africa (not to be confused with the waxy solid also called paraffin wax or just paraffin, or the much more viscous paraffin oil used as a laxative); the term kerosene is usual in much of Canada, the United States, Australia (where it is usually referred to colloquially as kero) and New Zealand.[3]
Kerosene is widely used to power jet-engined aircraft (jet fuel) and some rockets, but is also commonly used as a heating fuel and for fire toys such as poi.