Is there a way to write slanted non-italic (especially bold/bb lowercase) letters in math mode?

If you want the bold version slanted

\documentclass{article}
\usepackage{amsmath}
\DeclareSymbolFont{slant}{OT1}{\familydefault}{m}{sl}
\SetSymbolFont{slant}{bold}{OT1}{\familydefault}{bx}{sl}
\DeclareSymbolFontAlphabet{\mathsl}{slant}
\begin{document}
\[
fgh\mathsl{fgh}_{fgh\mathsl{fgh}}
\]
\[
\boldsymbol{f}\boldsymbol{\mathsl{f}}
\]
\end{document}

If you want the bold version unslanted

\documentclass{article}
\usepackage{amsmath}
\DeclareSymbolFont{slant}{OT1}{\familydefault}{m}{sl}
\SetSymbolFont{slant}{bold}{OT1}{\familydefault}{b}{sl}
\DeclareSymbolFontAlphabet{\mathsl}{slant}
\begin{document}
\[
fgh\mathsl{fgh}_{fgh\mathsl{fgh}}
\]
\[
\boldsymbol{f}\boldsymbol{\mathsl{f}}
\]
\end{document}

An alternative to Steven’s answer:

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{amsmath,amsfonts} % compatibility test

\DeclareMathAlphabet{\mathsl}  {T1}{cmr}{m} {sl}
\SetMathAlphabet{\mathsl}{bold}{T1}{cmr}{bx}{sl}



\begin{document}

Normal: \( \mathsl{ad}-\mathsl{bc} \).

{\bfseries\boldmath Boldfaced: \( \mathsl{ad}-\mathsl{bc} \).}

\end{document}