I doubt if you have a problem. Decreased mileage would be hard to pin on a smaller oil flow rate. Even if you had a slower flow rate, it is not likely to have caused any damage, unless it was enough to trigger the oil light.
Your message has brought up two additional issues.
First dealers are no better than other sources of car car over all and they tend to be more expensive. Second All quick lube places should be considered guilty until proven innocent, because they usually are. Don't visit them, even to ask directions we see all kinds of problems from them.
Good Luck