my understanding is petrol provides zero lub for the engine..all it does is provide some protection for the valves/seat bases due to additives...
having said that there are reasons why one should start on petrol when engine coolent cold..and thats vaporiser freezing up..and far too rich a gas mixture..
i have wideband o2 meter installed and it DID show off the scale richness if starting on gas untill coolent warms up abit...
however since i running full sequential petrol and gas with both controlled by megasquirt 3x i have the faclity to weaken gas injection times in relation to gas temp to keep it at 15.6 or so..plus of course i run varstly different ignition timing curves for each fuel..
still it dont run happy on gas till vaporiser is warm, i give it a mile or ao in winter, but in summer no probs..
i find open and closed loop systems are very different, open loop is much more tolerent of cold starts as long as back fires can be elliminated