Can additives e.t.c. Be added to quieten/ erase the sound from a clunking gearbox?

I drive a 5 speed mk2 2006 ford focus sport 1.8 TDCI

It goes into each gear perfectly with no hesitation at all, however it makes a clunk sound r change, thing it is only noticeable in low gears up to 3rd.
He says it is nothing to do with the clutch, not the release bearing or anything like that, he said it was (forgot what he said) inside the gearbox causing the clunk noise

Somebody said by adding additives to the gearbox will erase the clunking noise and make it go away.

He recommended slick 50 gearbox additive.

is he right that by using this the clunk will do away?

thanks

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