The Sindarin number “eight” derived from the root √TOLOD, probably from primitive ✱✶tolodō, where the [[s|[d] became [ð] after a vowel]] as usual.
Conceptual Development: The earliest attested word for “eight” was G. uvin in the Gnomish Lexicon from the 1910s (GL/75), replacing rejected ung. In The Etymologies from the 1930s it became N. toloth from the root ᴹ√TOLOTH, similar to but not quite the same as ᴹQ. tolto from the root ᴹ√TOLOT (Ety/TOL¹-OTH/OT).
In some notes from the 1950s, Tolkien used S. tolod (PE17/95), apparently deciding both Quenya and Sindarin were derived from the same root √TOLOT. Later still, toloth reappeared, but it was rejected and replaced by tolodh (toloð: VT42/25, 31). When Tolkien revisited the Elvish number system in the 1960s, he changed the t to a d in the root form for “eight” (VT47/11) and established tolodh as its Sindarin form (VT48/6).
Neo-Sindarin: I personally prefer tolodh as the Sindarin word for “eight”, but some Neo-Sindarin writers use the older (and perhaps better known) toloth. It seems Tolkien had considerable trouble deciding on the primitive root for “eight”, so any of these forms could be valid (VT47/31).
Tolkien emended toloth to tolodh, cf. VT/42:31 (and also VT/48:6). If we are to follow him, a word such as tolothen would be incorrect, unless the two forms coexisted