Leanest AFR I can get away with?

[fowljesse]

I know you can't tell me exactly, but I was wondering what the leanest people have gotten away with. I searched the forum, but didn't get enough info.
I tuned my engine management system to run lean at cruising scenerio; low manifold pressure at 60-65mph. I use high octane because I have 10.5:1 pistons. At this speed, I was about 17.5:1. That scares me. The car ran fine, and the EMS has a knock detector/ retard. It didn't get hot, according to the coolant temp sensor. However, I don't know if I could be melting the pistons, and not knowing it. When I give it gas, the AFR jumps to 13:1, depending on load.
If it helps to know; it is a short stroke 2.5L V6. There's no way I could hear knock if it happened, because the exhaust is loud.
Any ideas, AFR #s that have been high, but okay?