I've noticed the Oil tool has a weird presscurve at low sizes.
I've tried to take oil with RAZ settings, then with a size of 1, 5, 10, 20 and 30 (the default value), and here is the result:
You can notice at size 1 the line is the same with low and high pressure, at 5 size doesn't change but opacity does, and at 10,20 and 30 I can see some kind of steps in the presscurve.