@HA001 not really my theory, but it's a really good one by Yuderon, and what we sre seeing seems to suggest it's something "like" that.
Have a watch:
Oda doesn't seem to be going the natural rubber/amber route based on the heat steam we see and the fact that Luffy quite literally looks like he's melting in a way.
I do believe it's Oil at this point. The fact that he can ignite his fists too sort of implies he's been doing some version of it for a whil, or at least a loose implication of it.
I think he's going to legitimately generate explosions now with his fists.
@Sentinel came up with good reasons for why his attacks may be named after artillery/bombs now.