Lewy's example and most other allied examples of non-solvable operators have only complex coefficients. Leaving open the question of unsolvable operators with real coefficient. F.Treves in 1962 has constructed a fourth order operator iterating Lewy operator.
If $P$ is Lewy operator then $PP\bar{P}\bar{P}$ is a real coefficient fouth fourth order operator which is not local solvable.

