Mixing solvents, or how does paint thinner work?
I had dissolved some polycaprolactone in that universal thinner above, and it seemed to dissolve quicker than in either toluene or xylene alone.


Some of these universal thinners do contain a lot more than just toluene and the xylenes.

If you want to find out about the composition of these mixtures, a material safety data sheet (MSDS, in German: Sicherheitsdatenblatt), provided by the manufacturer gives some information:

As an example, take this MSDS by a swiss manufacturer, which lists the following ingredients:

\begin{array}{lr}
\textrm{compound} &amp; \textrm{amount [%]}\\
\hline
\textrm{toluene} &amp; 25-50\\
\textrm{xylene} &amp; 10-25\\
\textrm{acetone} &amp; 10-25\\
\textrm{propan-2-ol} &amp; 2.5-10\\
\textrm{4-methyl-pentan-2-one} &amp; 2,5-10\\
\textrm{2-methoxy-1-methylethylacetate} &amp; 2,5-10\\
\end{array}

Particularly the latter two might be much better solvents for the polycaprolactone than toluene or xylene.