How do you use both curved and straight voltages in LaTeX with circuitikz package?

Currently I am using:

\usepackage[european,straightvoltages]{circuitikz}

and in the body I have only:

\begin{circuitikz}
    \draw
    (0,0) to[R = $R_6$, v = $U_{R6}$] (1.5,0)
          to[R = $R_6$, v = $U_{R6}$] (3,0)
    ;
\end{circuitikz}

Please do not care about the naming. I just need to have both straight and curved voltage in the same file in LaTeX.

There is the whole code if it helps:

\documentclass[a4paper,11pt]{article}
\usepackage[european,straightvoltages]{circuitikz}

\begin{document}
    \section{My header}
        \begin{circuitikz}
            \draw
            (0,0) to[R = $R_6$, v = $U_{R6}$] (1.5,0)
            to[R = $R_6$, v = $U_{R6}$] (3,0)
            ;
        \end{circuitikz}
\end{document}

You can use voltage=straight to change the style of individual elements:

\documentclass[a4paper,11pt]{article}
\usepackage[european]{circuitikz}

\begin{document}
    \section{My header}
        \begin{circuitikz}
            \draw
            (0,0) to[R = $R_6$, v = $U_{R6}$] (1.5,0)
            to[R = $R_6$, v = $U_{R6}$, voltage=straight] (3,0)
            ;
        \end{circuitikz}
\end{document}