I'm about to try to refill a strut and found in a MAC-post that I have to use 320 ml. I have collected the oil from 3 struts in total over the years and filtered it very recently so it should be sludge-free - I want to use old oil so it will be matched in consistency with the other strut on the car which hasn't been repaired yet. I can quietly borrow a measuring cylinder from somewhere but I was wondering if I simply have to fill the strut to the brim and refit everything without too much spillage.... I suppose there should be no air inside it or can it tolerate a bit of atmosphere in there ;-?

You definitely don't want to fill to the brim! - stick to the correct measure.

The oil "filled" damper works by forcing oil through a restrictive valve (to provide the damping resistance) but in doing so creates heat (kinetic energy conversion to thermal energy which is radiated to atmosphere - in doing so kinetic energy is lost from the system and the free oscillation of the unsprung weight is reduced).
The thermal energy produced causes an expansion of the oil medium within the sealed damper unit and as oil is non compressible the increase in oil volume is taken up by an air space (air of course will compress). Without an expansion space the oil would simply be forced out of the damper past the piston rod seals.

Note also - "gas filled" dampers still use oil as a damping medium but the expansion space is charged with an inert gas, or more usually nitrogen.

Mac.

Many thanks for your superb advice... This topic reminds me of one of my triumphs of wit (or lack thereof).... Someone (who always values their street-wisdom) told me that the band 10cc was named after the average male eja..... To which I replied 'How should I know, I've never done it with a measuring cylinder'. Having said that I did find one in the garage today and its spring!