There are several different blade oils available that cause more damage to certain classes of monsters, e.g. spectre oil, arachnid oil, necrophage oil and insectoid oil.
Most of the monster classifications are pretty straightforward, but I don't know the correct classification for every monster for sure.
So, which monsters belong into which monster class?
Best Answer
If you press "C" for Character, then select Attributes at the bottom right, then Knowledge is the middle of three icons. Under that knowledge panel it lists the knowledge you'll find more detailed descriptions (if Geralt knows them, via reading books or experience fighting).
It will sometimes reference specific blade oils to use (for example Wraiths mention Specter Oil) but unfortunately still no clear categorization - I'm pretty sure Endragas are Insectoid, but their entry doesn't say so, and Necrophages is listed as a single entry, with mention of Rotfiends in the description rather than the name/category.