There are a number of issues with the oil brush: blending divergence and saturation artifacting, paint loading affected by zoom, loading having no effect whatever on wet into wet blending, loading generally not conserving volume of paint, back and forth strokes unexpectedly flipping brush (in real life one simply lifts the brush and places it back down for the next swipe), shape of brush contact not rendered properly etc.
This last one might be what you are encountering. For the oil brush, in the middle of any stroke, the calculation for how paint is laid down and interacts with the canvas is based on a 1D line at right angles to the stroke and having a width based in the "shape" selected and the aspect ratio setting. In real life a thin rectangular brush can be dragged at an acute angle with respect to its orientation (and keeping that orientation fixed), to form what roughly looks like a parallelogram. Try it with the oil brush (make sure rotation is controlled by your stylus so that it is independent of the direction of the stroke, controlled by stylus tilt only , or if you have Wacom's art brush, rotation of the stylus)
I had previously posted something in a thread about this issue but I cannot find it... bottom line is ArtRage oil brushes do not in fact have square or circular brush contact during the middle of a stroke, it is a line at right angles to the stroke.
EDIT: posted new thread in Windows OS