I don't know if there's any truth to it, but when I leveled synergy I tried to keep the synth level close to my own skill level. Skillups just seemed to roll in faster that way (like with fishing). Bump up to the next leaf synth when you're only getting 5% impurity per feed.
The reason the Leaf synths work so well is because they don't have a requisite craft skill to have beforehand.
Synergy furnaces will work outside, but I cannot attest to the skill up rate in Adoulin as my Synergy was capped long ago.
To my recollection, thwacking the furnace only produced a skillup if you successfully stop the explosion.