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Can I use denatured alcohol, scented bioethanol fireplace alchol (also deatured) all of which are much cheaper than non-denatured ethanol?

Is 90 % recommended?

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The additives in desaturated alcohol my influence the flexibility of the preserved specimen (though specimens in ‘pure’ ethanol also become stiff) but it may also render them unsuitable for later DNA analysis. DNA also degrades in 70% ethanol, but not too quickly.

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The mix I have is 90% ethanol, 5% methanol, 5% n-propyl acetate. Does the methanol or n-propyl acetate damage DNA?

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I hope someone in the know will tell us.

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I am pretty sure that unless there is some water in the mixture, the problem of hardening of the flies due to dehydration is inevitable. If however there is water in the system the DNA will spontaneously hydrolyse over time to some extent. I am not sure if this is still enzyme mediated reaction or not.

If I understand you, I should use pure alcohol, rather than denatured one.

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If water damages DNA but methanol, propanol, and similar organic compounds do not then denatured alcohol should work. I don't know if that is true. But beware that some chemicals will absorb water from the air.

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I did some research.

The problem with denatured alcohol is that it contains a variety of additives that makes the alcohol unfit for human consumption. But it seems that it is not made public what is added, most likely to prevent others to try to remove them and than make money on selling it for consumption. Some of the potential additives may be problematic. If the actual composition would be provided, maybe some denatured alcohol could be used. (For example if "denaturation" is made by iso-propanol and or ethanol).

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I asked somebody who works with DNA. An enzyme degrades DNA in the presence of water. She doesn't think the type of alcohol matters as long as it is nearly free of water.