So today, I am researching the differences between these oil type. According to this article from the Dept of Forestry in Indonesia, MKP comes from a variant of genus that includes Myrtle tree, and its export is governed by the strict law. While Eucalyptus originated from Australia, been used globally. Hmm since both trees are originated from very closeby location (the Austro-Indo group) and have very similar smell, I suspect more connection.
Here's pic of my favourite- the best brand of MKP - is Cap Ayam from Medan, North Sumatra! It's advertised by local online shops at 150ml for 38.000 Indo rupiah or around S$5.
Memory of childhood: minyak kayu putih cap ayam, bikinan Pt Jamu Sari
More research to come.