Thank you for your feedback!
I unfortunately ran out of time and couldn't make a proper introduction to how different tiles interact with each other (so there is just the quick overview in the description). The reason why water sometimes didn't spread over dirt was because it was fighting for that tile with the fire, in 1v1 (water-dirt-fire) situation it is a draw, you need more water tiles surrounding (not diagonally) the dirt block than there are fires to claim that tile as water.
Your observation about the oil was correct, you could think of this as a DPS difference, for example fire deals less "damage" to wet (dark green) grass than to regular grass, and oil spreads over water faster than water spreads over regular grass