A highly scientific, well documented comment:
Service manager at a shop just got a case of AMSOIL SS from me. I asked if he had used it before, in his Mercury with about 150,000 miles. Answer was yes, he had. Then he cheaped out and put in regular. On the same trip he had gotten about 26 mpg with AMSOIL, he got about 22 MPG with the other. In 10,000 miles, that would be substantial $$$ in his pocket. No, I don't know what other oil he used.