If it says not to change it- don't. Usually the system on machines that say not to change the oil is sealed, and opening it will alllow contaminants in or leak points to start. I had a Toro garden tractor that had a sealed Tuff Torque trans that contaioned 10w-30 oil. It looked black to me too, and I was so tempted to try to change it, but it ran so well and functioned so well that I never tried.
The fact that it ran 30 minutes with you off the seat makes me think that it may just be the seat's kill switch acting up. You can't always tell by looking at it. Before you do too much, try to ask your dealer tomorrow and tell them exactly what you've experienced so far. They may be able to check your seat switch for you.