Changing hydraulic oil?

[PeterBergin at aol.com]

My steering rack leaks a small amount.  So I stopped by the local dealer to 
buy some Pentosin.  Ask for specific oil for my car and was sold the old stuff 
by mistake which I added unknowing.  

So now I have two cans of the correct synthetic Pentosin to change out the 
system.  However after thumbing through my Bentley, I haven't found information 
on how to change out the old Pentosin and how much the system holds.  Also 
bought some hydraulic oil stop leak from Blau. to add to the system.

Can any one give me so quick advice?

Pete
91 Avant