Very stupid question, astra GTC 1.4T, what fuel?

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and no I don't mean petrol or diesel,
I mean that I got this car a week ago and coming from a thirsty 1.8 insignia without thought I loaded it up with Sainsbury's cheapest fuel and it doesn't quite feel as strong as when I first picked it up with that tank of gas from the dealer.

I have already looked in the manual, fuel filler cap and online for any suggestion but found nothing.

Any suggestions for what grade of fuel I should be using? worried about screwing up the turbo