\"Engine Must Be Run On 91 Octane Fuel\".....Do I Have To? I noticed a tag when I pulled out my 500 (which I found out was actually a 472cid), that said the engine must be ran on 91 octane fuel. 91 octane in canada sure as hell aint cheap here, can I get away with regular 87? The motor was built for propane, so it has hadened seats, so I imagine it currently not pining due to the advance. Any insight on these old motors?