A six on a five should not break a rod. It is best in my opinion is to cast several lines to find what works for that rod and YOU. I have several rods that folks say work well with a 5 while my casting stroke says 6. The reason for a Sylk line is that it is smaller in diameter to better fit through the old small diameter guides. If you rod was built recently any new line will work. Many folks like a Wulff tri angle taper. Then another like Cortland 333 that is also a thin line.

Folks can't say what will work best for you. I use real silk on all of my cane rods, though I have a DT6 Sylk for my 7 ft Battenkill. I have a new rod that is a clone of a Leonard 39 7 1/2 ft and it cast a 6 or 5 well but it will not cast a 4 at least for me.