I have trouble when reading Russian articles that use metric altitudes, always trying to convert it mentally. With imperial and metric, I'm all over the place.
I think of surface distance in kilometres, building measurements in metres and millimetres (not cm), depth in feet, area in acres, pressure in psi, volume in litres; it can get a bit confused.
edit:
When it comes to altitude and oxygen requirements, no two publications ever give the same height exactly. Russian manuals are a bit out from Western ones as they tend to round off to the nearest 1,000 metres which is quite a bit of leeway.