What level does the oil quantity on the cockpit gauges need to drop below before you need to add oil?
How much canvas can show on a tire before you need a wheel change?
What reference do you use to check this?
1. We don't know. It's not in the documentation.
2. We don't know. It's not in the documentation. I asked an engineer once and he told me, but I still call an engineer to verify tyre damage when I see it because "some guy told me" is not something that will stand at the subsequent inquiry.
3. If in doubt call Maintenance Watch. If not satisfied with their answer, we still have the legal responsibility not to depart until we are completely satisfied with the airworthiness of the aircraft. If that results in tea and bickies then so be it, but I've never heard of a pilot in my company subject to disciplinary action for grounding an aircraft.