The 1.8-litre non-turbo and 1.4-litre turbocharged engines in the 2014 Cruze will run happily on 91-octane petrol. The more highly tuned 1.6-litre turbo engine in the SRi, SRi V and Z Series models required a minimum of 95-octane fuel. There's no problem using 95-octane fuel in the engines designed for 91 fuel, but you probably won't gain anything in terms of fuel consumption or performance. The only thing that will happen quicker will be the emptying of your wallet. There's no problem mixing 91 and 95-octane fuel in the cars designed to run on 91-octane petrol, but the 1.6-litre engine can be damaged by using anything other than a fuel with a minimum octane rating of 95.