When I first got my IR I still had my 152 and did quite a few airways flights in 10-12k feet. (Fully IFR equipped, GNS 430, ADF, DME, Dual Alt etc.).
IO540 is quite right that if someone is feeling noticeable ill effects at 8,000ft they have a problem. Long periods at 10,000ft without out will leave you tired and more difficulty concentrating but are not dangerous. It is only this year the CAA have started to mandate the use of O2 for the PILOT above 10,000ft. I do not put my passengers on O2 until 14,000ft.
My average sats at 10k without O2 are high 80's on O2 high 90's.
My Cessna has an 18,000ft ceiling and I have often had to get up there to avoid weather in France. 200fpm climb rate up there!