Premium.. the higher octane (unleaded, in this case) option found at the pumps of most petrol stations (here in the UK, at least).
I've had cars previously that, in spite of running a factory map, have benefited from higher octane (98RON iirc) fuel. And each one 'preferred' a different brand of said fuel.
My Cav V6 would run best on Shell Optumax, for instance (and I would know within minutes if the Wife had put in regular unleaded or non-Shell fuel)
My GSi Carlton (12V) ran best on Esso Super Unleaded.
The 24V Senator was an Esso Super car too.
Just wondered if folks running the Z22SE had similar findings (or not).