Thank you very much for your input,Big Mick et al!
Presently I do use Thomastik-Infeld produced in Austria. Not bad at all, but I'm not quite sure whether they are really suitable for a Guild F 312-Special and for playing the blues. Moreover, some times I do use a plectrum.
I believe it's most difficult to find the "right" product, because one has to find/make a compromise between a special guitar-brand, kind of music to be played with, technique of playing an instrument, price aso.

In former times I made some good experiences with Martin, Darco and-if these have been for my Guild, which I cannot remember exactly - La Bella.

Again, thanks for your recommendations! Will remember all of them when there's a need for new stringing,

Manfred