I'm with the other guys.
I have been wet shaving for many years.
I have never used shampoo on a brush.
Think about it. If you soap the brush up, then thoroughly rinse, does it really matter if you are using shave soap or shampoo?? It ends up clean.
When I get a new brush I will do a lather and rinse before my first shave which is good enough for me.
Pete <:-}