Anything over the manufacture recommended oil change interval would be considered a extended OCI, to me it's that simple, you can not put a mileage on it in a general anything over xxx covers this criteria, look at my Honda, normal OCI using regular old conventional is 7500 miles, so a extended OCI for me is around 15k ( I try to hit between 12k-15k) I think when you start looking, and performing the Amsoil extended OCI's then your jumping into a different league of extended drains, which from reading I have seen many use a by-pass oil filtration system, and also use oil analysis as a tool for monitoring the condition of the oil, us normal (broke) people trust that the synthetics (I'm guessing over 90% of that us is using a real world hydro-cracked dyno oil with a high additive package for extending the OCI)