I think oil volume is dependant on the weight of gas used in the system. So the same compressor used on different vehicles will require variable amounts of oil.
When you have the system re-gassed the machine will add the correct quantity of oil. This will also fully test for leaks and remove any moisture that will present due to the system being opened to the atmosphere.
I went through several springs both MB and after market before fitting Eibach. The problem with after market stuff is ride height, claims of OE spec are often not so and you finish up with it too high.