As you say, in the U.S. I believe the standard is by weight and in Europe by volume (in general). But even if most engine brands come from Europe, most of the fuel come from the U.S.
I unfortunately can not find more than one brand at the moment and that site seems down for the night... http://www.gt-racing.de/
I will get back when my memory serves me better.
But if we toy with the idea of mixing fuel with 25% Nitro and 10% Oil, and use a density of 0,79g/ml for methanol, 0,95g/ml for oil and 1,138g/ml for nitro then that would look like this:
1000gram = 1147,72ml
Methanol 65% =650g = 822,8ml or 71,69% by volume
Nitro 25% =250g = 219,68ml or 19,14% by volume
Oil 10% =100g = 105,26ml or 9,17% by volume
1000ml = 893gram
Methanol 65% =650ml = 513,5g or 57,5% by weight
Nitro 25% =250ml = 284,5g or 31,85% by weight
Oil 10% =100ml = 95g or 10,63% by weight
But as you say, they probably donΒ΄t mix oil by weight, my guess is on the heavy expensive ingredient.