I recently aquired an Orvis bamboo rod it's a Battenkill 6 1/2ft 2 5/8oz and has a 6 in parentheses which I assume is the suggested line weight. To me a 6 weight line would be a little on the heavy side for this rod. I've fished it with a 5 wf orvis line and it casts ok but I'm in need of some new lines so I was wondering if anyone had any suggestions. I've been considering the SA 4 or 5wf because the SA lines are half size larger.

I have a similar rod, although I want to say mine is a 7' 6 wt Battenkill.  I've used an old 4/5 wt triangle taper on it and thought it worked pretty well.  I purchased the 4 wt triangle taper last year to replace it but  I haven't tried it on the bamboo yet.  I went through a couple lines a season or two ago to match up to my old 5 wt graphite.  I tried the Orvis DT, the Rio which Kieth now has and ended up back with the Wulff line.  I really like it.  I think my father used to fish a 6 wt cortland line on the bamboo.