I have never heard of an LED wearing out (unless "failing" also counts) ... but lenses can become scratched, "fogged" or otherwise compromised - as with a residue as Dustcap has suggested.
When you say "continuous use" do you mean continuously on the same patient?
When you compared with other monitors ... what did you find? Were those results within expected limits?
I seem to recall that SpO2 monitors in general would be notorious for over-estimating on dark skin; but 10% sounds a bit high (I think that 2% was regarded as typical). But I would expect that - even using the latest types - results may vary between the various technologies (algorithms, interpretations of R-curves).
