This we did as an experiment on an E32 750iL Alpina B12. In general it is said the shocks are non servicable, but it is possible, we already had some apart from the rear and replaced some o-rings. Front shocks are a bit more difficult, but this time we welded 2 washers to the top plate with an auto body stud gun welder and then used the dent (slide)hammer puller which comes along in such a set. Now it was easy to lift/pull/hammer the top seal out without damages, like this
http://8810.teacup.com/750hiline/img/bbs/0001321M.jpg
http://8810.teacup.com/750hiline/img/bbs/0001321_2M.jpg
http://8810.teacup.com/750hiline/img/bbs/0001321_3M.jpg
In case someone is wondering why the wheel is still on (or again on), that we did re-attach to have the shock nicely secured and have power when using the slide hammer.

The oil inside was quite dirty from >20 year or working and wear tiny material from the seals was found when we filtered it and it looked similar like Pentosin. We measured the drained oil and replaced it with the same volume of about 400 ml. We used oil for motorbike shock absorbers and so far so good. Were also thinking about forklift oil
From what I have got as info from someone who is also studying this, the original fluid/oil is

ESSO EGL 45 374 or DEA 5045 f (the f stands in a circle O)
volume/quatity:
The piston must be in oil up to the upper rim of the cylinder (a small amount of oil must flow into the spare/reserve area between inner and outer cylinder when closing the cylinder, but the spare area should not be filled more than 2/3 filling height. Pictures or drawings I unfortunately cannot send you. The sealing cap of the front shock probably must be destroyed, for example by heating, the inner gaskets melt. But you must keep them, as most of them you have to re-use.In case you find a good way to add compressed air, please let me know., thank you. 3 Bar