In my experience, the pulse oximeter was a bit erratic. You need to be careful placing it correctly and consistently. OTOH I suffer from Raynaud's, so blood circulation to my fingertips is not ideal.
Very much so. I can easily get successive readings between 91-98% with different fingers.
Partly it is the old story of needing to become familiar with your equipment's limitations. I find the most repeatable measurements are if I don't let it clamp down tight on my finger.
Nonetheless, for this purpose poor repeatability is acceptable: if the reading remains below, say, 89%, then get proper advice.
Here they have been issued to people whom are more likely to suffer problems as a cheap sanity check. I haven't heard anything about the results, but I doubt it will do much harm.