True, unpressurized aircraft are allowed to sail along at 10k feet without supplemental oxygen, however, it is certainly mentioned as a worthwhile option.
As low as 5,000 feet, decreased oxygen can affect the night vision of even young, fit, healthy non-smokers. The effects on the body of 8,000 feet are there, even if they’re subtle in most people.
On an airliner you’re generally sitting in one spot and not exerting yourself - walking around, up and down hills and stairs, at 8,000 feet is a different matter. Altitude tolerance varies widely among people, and it isn’t always a matter of the athletes doing better. I’ve noticed a descreased tolerance for exertion at 5,000-6,000 feet. I can run - just not as fast or far. I can go up and down stairs - but only about half as many as I can at sea level.
So… pace yourself for a couple days. DO drink lots of fluids but avoid the alcohol and other diuretics. I don’t know what the danger signals are for truly serious altitude problems, but I’m sure their available on-line or, better yet, as others have suggested, ask a travel doctor.