Hello Kenmo:
This is not really a "proof", but there are at least three reasons why the new custom brush with depth effects is better than the standard old AR Oil brush.
In the picture below the oil brush is shown in the top half, the custom brush with depth and blending effects in the bottom half.
1. Brush strokes and dabs are much more varied and organic looking with the custom brush. Brush stroke shapes, and textures for the old Oil Brush are much more regular/digital in appearance. Also note, with the old Oils only dab or stroke are possible, and not anything in between.
2. Tight circles or aggressive direction change in the Old oils break down, producing broken strokes (even when the stylus is not lifted) and digital "cornering" which can be visible. Custom brush allows complete control with no breakage and no discernible little digital "cornering" even for tight circles.
3. Blending with the standard old AR Oil Brush can lead to divergent colors, i.e. over saturation, which I have been a little "cantankerous" over on this forum in the past. The custom brush can be used to blend color as much as you want and the color never diverges, it always "converges" becoming some color in between those you are mixing. Blending with the custom brush is a joy.
IMHO The new AR6 custom brush with depth effects, once mastered (it does take work to build and tweak good brushes) can be far superior to the standard old AR Oil Brush.
Cheers!!