I go to about 15% as the lowest. Usually I change around 20% +/-. Its definitely black when I change it lol, but I think Amsoil guarantees it to 25K under “normal” service and 15K under severe.
I used the same oil in my STi with an Amsoil Maximum Efficiency oil filter and I never had an issue with oil consumption. I changed that every 6K due to fear of potential issues with the rings and lack of OLM.
Heres one study Consumer Reports did on synthetic oils: http://www.ganoa.org/Motor_oil2.htm