You should not need high octane fuel in that mower engine. One point I should explain, though. Honda's pushrod overhead valve, and overhead camshaft, engines (yours is pushrod) have much higher compression ratios than both Honda and Briggs & Stratton side valve engines (either 8 or 8.7 to one compared with 6 to one). Hence there is quite a lot of nonsense you can get away with on the side valve ones that is not to be considered on the OHV or OHC ones. Also this is why your small engine (160 cc) has an automatic decompressor, which small side valve engines do not.
Honda specifies at least 91 Research Octane Number (RON) fuel for your engine. That is the specification of standard unleaded in Australia. Premium unleaded is 95 RON. Use of Australian premium unleaded fuel in your engine should not cause any problems but is not required and is unlikely to produce any benefits.
