But surely just because a machine recovers no oil it doesn't necessarily mean there was none in there? The oil would have to evaporate under vacuum conditions for the machine to be able to pull all the old oil out.
My take on the machines is that they are there to pull old gas out, vacuum test (which will also boil off water) and push new gas in... but they have to be able to cope with some oil. No easy way of getting old oil out? It wouldn't be much good just sticking X amount of new oil in the system on every fill or eventually after enough refills the AC system would contain just oil and no refrigerant.
Analogy - Could remove an engine's oil filler cap and shove a vacuum cleaner pipe in the hole (maybe as part of some PCV valve test or something), would expect the vacuum cleaner to pull some oil out but what force is the vacuum cleaner providing that could possibly get all the oil out of the engine including from the sump?
Think I'm going to just have to guestimate it... I will have added some extra oil when using one of those Halfords DIY regassing kits (some oil in those bottles), question is was there the correct amount of oil in the system before I used the Halfords kit considering my system had a leak at the lowest point of the system and some of the AC pipes are a metre or so above the lowest point.