I'm having a second look at oil pastels just now. Bought student grade originally and didn't like it much. Since then I decided to invest in a few Sennelier and Caran d'Ache. Low and and behold, that's an interesting medium and it plays well with gouache.
I understand Mungyo Gallery are also pretty good, but no open stock available in Germany.
Although, I think time is starting to run out for plein air painting this year, so maybe it might have to wait until next year to try them out in the field.