How to make a symbol more boldface within a boldface section heading

I have the following section heading:

\section{The system M of Minimal Logic}


which of course is all rendered in boldface in the pdf. How can I make the "M" even more boldface within this heading?

One way to do this is to use a font with more weights than just regular and bold. One such font is Adobe Serif Pro. But there are many and some have packages. If there isn't a package, then I'd use fontspec since it's easy to load arbitrary fonts.

\documentclass{article}
\usepackage{sourceserifpro}
\pagestyle{empty}
\begin{document}
\ifxetex
\section{The system {\sourceserifproextreme\bfseries M} of Minimal Logic}
\else
\section{The system {\fontseries{k}\selectfont M} of Minimal Logic}
\fi
This is regular weight.
\end{document}