Kudos to your brother for putting gas in the tank!
I can't even imagine putting regular gas into a turbo motor. The engine controls will keep harm from being done (short term), so you could use it in a pinch.
It does handcuff performance, though.
Even my NA 128 is set up for premium gas. Inside the fuel filler door, it does say "91 octane recommended." I haven't used anything else.