A buddy and I have been fighting over which is best for summers over the last few years between 8lb leader and 6lb leader. Both of us agreed there was no reason to use 4 because fish need to be brought in quick and not lactossed up and exhausted. We have both hit the century mark on summers each of the last few years with me using 8 and him 6. So both are pretty much equal, with a few twists...

The advantage of a thinner diamator line(6)is it allows a more natural presentation if your float fishing, meaning your bait sinks faster and your jig whisps around more. and for drift fishing your corkie will ride higher in the water with a thinner line(less weight, more boyancy)

A little bit thicker line(8lb) is nice if you need to turn a fish from going over a tailout or getting hung on a log/branch.

I spin fish 90% in the summer, so low memory main lines are a must for me. Try 10lb Trilene Big Game and 6 or 8lb Max Ug...Stay away from Izor 8 and 6. I learned the hard way. 12# izor is tough stuff, not the greatest mainline.

I know some guys who use 12lb mono in gin clear and do fairly well, so to each his own... If the fish are in fast riffley water were reaction time is fast, you could probably get away with 15# leader. 10 and 8/6 is good all around summer setup
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"I have a fair idea of what to expect from the river, and usually, because I fish it that way, the river gives me approximately what I expect of it. But sooner or later something always comes up to change the set of my ways..."
- Roderick Haig-Brown