If it was my car, I would get the oil analyzed at 10K if I was going to go that far or more. That's the only "evidence" you're going to find. Otherwise, you wouldn't know when the oil was degraded to the point of damaging your engine. While M1 may be able to protect Engine A for 10k, it may only be enough to protect Engine B for 7,500.
Pennzoil Platinum is a cheaper and I think better alternative to M1. A quick search on the BITOG forums will answer any questions.
Boutique oils like Amsoil (which I'm currently using) have specific oils meant for extended drain intervals. The one I'm using advertises 35,000 miles normal service and 17,500 severe service, though I take it with a grain of salt. It's quite expensive, and only worth the money if you decide to follow their schedule (in which case you would
save money).
Does your car have the same Toyota 1.8L 1ZZ-FE as newer Vibes and Corollas? Here is a
Used Oil Analysis of an '06 Vibe with 7500+ miles on Havoline synthetic. Results look perfectly okay.