They seriously need to buff/increase the drop rate for Purified Oil. I've killed about 168 3-Eyed Sumashi to get just 2 Purified Oil and another one after 225.

Maybe it's an ultra rare drop and you're unlucky.

Drop rate is like 1% - 2% from what I've experienced. Compared to other boss items, this is probably one of the most difficult one.
Asides that, there is a quest where you can trade 20 dark oil for 1 purified oil, but I used that one already long time ago...

Try farming for Centipede's Poison, there's 2 chances to get one in the whole dungeon and I have 400+ kills and have yet to get one lol. Now that's not a fun drop rate.

Centipede Poison.. I find that easier to get with full party running Hell mode