Hello LunatiqueRob and welcome to the forum. With the oil brush, if you vary the
Loading the stroke will end in a fade. The
Blender tool can be used to blend one colour into another. Try the various presets - there is one -
Just Blend Color which may be useful.
As for the Designer Brush - as long as you have
Head and
Grain selected the brush should show. It would help to know which settings you use. The image below is just random settings as an example.
If these are not the answers to your question I expect Markw will join in.