Ethyl alcohol is denatured, to avoid taxes. As such, in the US, the government regulates the formula for denaturing, and the standard formula's are listed in the "Code of Federal Regulations". SLX, was a designated formula from the prohibition days, although I forget what it stands for, but that would correspond to formula #20 today. Formula #19 is more commonly used, but still contains gasoline.
SD alcohol, is "specifically denatured", and would be an alcohol denatured using an alternate process, for safe use within some other product (like toothpaste, etc.).
Denatured alcohol you buy in the hardware store, must be denatured using one of the standard listed formulas, with any alteration (even adding odorant, or colorant), requiring approval from the ATF. See:
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr;rgn=div5;view=text;node=27%3A1.0.1.1.17;idno=27;sid=1ce323b41d8176068fcdb3baf5e431f8;cc=ecfr#27:1.0.1.1.17.3.237.1(edited above: I changed the link to one that points to the CFR in the National Archives website)
From CFR part 21:
§21.22 Formula No. 18.
To every 100 gallons of ethyl alcohol of not less than 160 proof add:
2.50 gallons of either methyl isobutyl ketone, mixed isomers of nitropropane, or methyl n- butyl ketone;
0.125 gallon of pyronate or a similar compound;
0.50 gallon acetaldol (beta-hydroxybutyraldehyde); and
1.00 gallon of either kerosene, deodorized kerosene, gasoline, unleaded gasoline, rubber hydrocarbon solvent, or heptane.
§21.23 Formula No. 19. [Probably the most common denatured alcohol sold at retail.]
To every 100 gallons of ethyl alcohol of not less than 160 proof add:
4.0 gallons of either methyl isobutyl ketone, mixed isomers of nitropropane, or methyl n- butyl ketone; and
1.0 gallon of either kerosene, deodorized kerosene, gasoline, unleaded gasoline, rubber hydrocarbon solvent, or heptane.
§21.24 Formula No. 20.
(a) Formula. To every 100 gallons of ethyl alcohol of not less than 195 proof add:
A total of 2.0 gallons of either unleaded gasoline, rubber hydrocarbon solvent, kerosene, or deodorized kerosene; or any combination of these.
(b) Authorized use. Restricted to fuel use, comparable to specially denatured alcohol "Use Code No." 611, 612, 613, 620, and 630.
Notice something interesting in here?
Methyl alcohol, and isopropyl alcohol aren't listed at all. They're not actually added to denature alcohol. They're just byproducts of the production of ethyl alcohol, which aren't removed (kind of a why bother removing them, since it's poison anyway, situation).
The dirty secret, is that when sugar is fermented, both ethyl and methyl alcohol are produced. Ethyl alcohol is the larger constituent, but the tiny proportion of methyl alcohol left remaining after distillation and filtration can be responsible for many hangover symptoms, and would need to be listed in an industrially regulated product.
Hola Giovanni!
