Marius,
I've been using the Red Top 25 for 2 seasons on my K. It works fine in the summer, no problems at all. Last winter it wouldn't turn the engine over for long enough to fire up - but I think that was probably because the starter motor was dying. I wouldn't rely on it to start in -10 Celsius, but then I prefer to be indoors in the warm when it's that cold.
Mike