Freshman need to locally adapt SIunits

Use the siunitx package. It is now the standard for typesetting units and numbers. Also keep in mind the the Scandinavian countries use the decimal comma as a decimal marker.

\documentclass{article}
\usepackage[output-decimal-marker={,},
            group-separator={\,},
            number-unit-product={\,},
            inter-unit-product=\ensuremath{{\cdot}},
            exponent-product={{\times}},
            separate-uncertainty=true
           ]{siunitx}
\DeclareSIUnit{\mil}{mil}
\begin{document}
\noindent
\SI{1}{\mil} = \SI{10}{\km} \\
\SI{1.23}{\mm}               \\
\num{1.23e5}                \\
\end{document}

You can also just write \SI{1}{mil}. I prefer the abbreviations = false option, because

    \SI{30}{W/m^2 .K}

has a lot less typing than

    \SI{30}{\watt\per\square\metre\kelvin}

Note: There is a locale option in siunitx, but none of the Scandinavian countries are listed. You can contribute to the community by compiling a proper setup for you country.

This should do the trick:

\documentclass{article}
\usepackage{siunitx}
\DeclareSIUnit{\mil}{mil}
\begin{document}
\SI{1}{\mil}
\end{document}