Hello Antony and welcome to the ArtRage forums 
The Oil brush lays down paint by default Wet. But if desired Dry by ticking the Instadry check box in the tool’s Settings panel.
If Instadry has been used for some brush strokes then those strokes will instantly dry on the canvas as they are put down and so be harder to blend.
But no type of paint in AR will dry over time. (Though some sort of timed drying is something that has been asked for in the past).
Outside of that there are several things that can also effect blending of any paint in AR;
Paint depth, paint type, Canvas texture, the pressure of the tool doing the blending, what type of tool it is; Oil brush, Knife, Custom Brush and the exact internal settings being used by said tool.
In short there are quit a few variables!
Maker Of Replica Macoys
Techie Stuff:
ArtRage 6.1.1 ~ 15" Macbook Pro ~ macOS 10.14.6 ~ 4 Core i7 3.1GHz CPU ~ 16GB RAM ~ Wacom Intuos4 M