Sure seems like we just covered this exact same baler and situation in another thread this week . . .
jmhinpa, In both of your posts, you expressed concern that your dealer "probably won't support" your baler if you switch to sisal. The sisal won't damage anything, so I think the worst that could happen with your dealer is he'd tell you to switch back to plastic if you have knotter problems. (He might say, "I told you so.") Otherwise the "support" will be the same. You won't be voiding any warranty, will you?
Around here, used equipment doesn't come with much of a warranty. If a dealer says it will tie a knot, they'd stand behind that but wouldn't really guarantee that something won't break next week.