Ken, the old standard-grade petrol in the leaded fuel days was 87 octane, and the Victas of that era ran perfectly well on it. Current standard unleaded is 91 octane, and that level is more than adequate for old Victas. Old B&S 4-strokes are in the same situation; their compression ratios are very, very low. However be careful with Honda and modern OHV B&S engines, which have much higher compression ratios. Follow the manufacturer's recommendations for octane rating.
Simple 2-stroke engines such as Victa made have rather low effective compression ratios due to leakage through the ports. They also have very poor scavenging, which means they retain a lot of exhaust gas - the same effect as exhaust gas recirculation on a car engine. It dilutes the charge, reducing burn rates and tending to suppress detonation. Engines of that type are seldom very demanding of fuel octane level - but be very careful with high speed, high output two strokes such as high performance 2-stroke motorcycle engines, and some chainsaws - follow the manufacturer's fuel recommendations closely with all types of engines.
