You can but you shouldn't use supreme unless the car is specified for supreme and if it is you shouldn't put regular in it.
Regular unleaded
Yes In a modern car changing from regular to premium is no problem It may have been costing you power and fuel mileage to run regular, but if your car does not need premium it is a waste of money. On older cars running regular may damage the engine, but we are talking cars built in the 1980s or older.
In 60's terms you could put regular or Hi-test in it. Leaded of course. Unleaded is supposedly hard on an engine not designed for it. There might be additives available to offset that.
Regular unleaded.
Regular Unleaded.
its not Recommeded.its a high proformance engine.
" regular " unleaded - 87 octane
" regular " unleaded , 87 octane
Nothing happens, except your wallet will be lighter.
" regular " unleaded - 87 octane
regular 87 unleaded is just fine
" regular " unleaded , 87 octane