butane propane mixture as for a bbq

Fuel gases are heavily contaminated and unsuitable for direct application to food. Here's a collection of statements about the heavy ends in propane fuels from the vendors:
Some of these heavy ends (especially if made from coal) can be aromatics like napthalene, which are carcinogens and do not burn as readily as short aliphatics like propane. You'll be painting your food with carcinogens. Only fools violate the rules.

interesting , I have not yet heard food contact as a reason, and I am not sure it is valid for the autogas supplied in Australia. I have forwarded a question to one of our suppliers so it will be interesting to see if that comes up. Only fools ( and lazy people) accept rules without questioning them..

Dear terry:
... If not, why not?
Only babies learn about the world around them by putting things from it in their mouths first...
David A. Smith

If food contact was such a critical issue I would expect to be able to find an Australian standard for propane for cooking use. I cannot ( which of course does not mean it does not exist) whereas I have been able to find a standard for automotive use. On no official site of gas suppliers in Australia have I seen a reference to not using autogas for health reasons.
I am not sure what you imply by that remark. I am certainly not advocating anybody try using autogas in a bbq, I am simply trying to ascertain for sure why it is illegal ( and I have confirmed that it is ) Real safety is obtained when you understand the science not the laws.

Dear terry:
... I imply that others have raised a question about the safety of the gas source's constituents as being safe for human consumption. Early in life, we have parents that make rules we have to follow, until they can convey (if they know) the reasons behind the rules.
You have implied exactly this. You want to know why the cheaper source cannot be used. If that was not your intent, then you did not say that in the first place. I understand you are expending considerable energy distancing yourself from that interpretation / intent. You don't need to keep doing this, as I'll quit bitching.
Most people are not capable or are not interested in understanding the "science" behind safety. That is why there *are* laws. In lieu of having a hungry mind such as yours appears to be, "real safety" comes from following the laws. You don't need to fabricate homilies to justify getting your answers.
There is an orifice in gas-fired appliances, that is sized for a certain heat rate. Its sizing is based on an assumption of the fuel being evaporated, and the heat rate the appliance can safely produce. Putting in a different fuel can: 1) clog the works, or damage the seals; 2) produce a very sooty flame, since atmospheric-pressure ignition temperatures might not be suitable for the mix; 3) produce too much flame, since too much fuel might be allowed to the burners; 4) more (such as contaminants, which I find uncompelling since all fuel sources are "dirty"); 5) not paying the appropriate taxes, allowing people to eat without Big Brother knowing they are, or more likely using subsidized transportation fuel for cooking.
I don't find #4 or #5 to be particularly compelling, but I do believe there to be "mechanical" reasons why it might be a bad idea to cook with a different blend of (eventually) gaseous fossil fuels.
In general, laws have some justification written into or recorded "adjacent" for their existence. I'd suggest you go to the text of the law, to seek out the basis of the law.
David A. Smith

yes and I appreciated that input and said that I would look into it, in fact I have already sent off an email to a supplier who sells both propane and autogas . And yes I agree babies and children should just accept what their parents tell them. but I am not a child, I am an adult and a trained chemist. I ought to be to know why the law is in place, and frankly I feel a bit embarrassed that I dont, especially considering I work in the petrochemical industry.
I think you have misunderstood. I am not distancing myself from anything. Of course I want to know why the cheaper source cannot be used, its a perfectly logical question to ask, My point was I would not advocate to anybody else when I do not know myself (yet) whether there is in fact a danger.
Most people are not qualified chemists and dont read this forum. I am really lost as to where you are coming from. it seems to me you are trying to create points of difference that dont exist in order to stir up an argument. I dont disagree that people should follow the laws, but law abiding people still get injured in all sorts of accidents every day. Many of these accidents could have been avoided by having a better understanding of the science of what they are doing. Thats all I meant to imply.
And people will always choose to break laws, if they didnt we wouldnt need police, At least if they understand the reason for the law they are going to break they are less likely to hurt themselves or others. For example if we just promulgate the idea that autogas cant be used for the bbq just because of a food contact reasons, someone might think it is therefore OK to use it for a gas fired lighting apparatus, and if in fact if it was the CO problem they are still in danger if they set the light up in their unventilated garage.
there are certainly heat value differences between butane and propane but these are small compared to the difference between say methane and propane , which requires the use of smaller jets in gas burning appliances. Butane requires 6.5 vs 5.0 parts by volume compared to propane for complete combustion so it is possible that more CO will be produced and a sooty flame if enough oxygen is not drawn in by the burner. but that is something that could be quite easily tested for ( I have access to CO measuring instruments) I have also read many personal accounts of people already using autogas for cookers etc with no apparent problems ( which of course doesnt mean there isnt any... most people would not have access to CO measuring instruments) and it could well be a significant problem if people were to use it indoors or without good ventilation.. but again I want to know for sure.
Tax is not an issue here, the high cost of propane vs autogas is related to the excess of butane available in Australian refineries and the much larger volumes involved with autogas compared to propane. I have also since ascertained that the standard for autogas in Australia dictates a vapour pressure btween 800 and 1530 Kpag at 40 deg C. This would allow anywhere between 0 and about 50 % butane in a butane / propane mix.
I have, but there is no basis given it is simply stated that it is an offence to fill a portable cyclinder with autogas. Thanks for your input.