You don't want more than 5 weight, particularly in the shock - otherwise you will be "hating life". I am not sure what oil the sachs shock takes - you should really talk to Les before you do anything - these normally take a spectro very lite or ultra lite shock oil - I think the ultra lite is a 2 1/2 weight-ish oil...
In reference to the leaking reservoir piston - I asked Les at LT-Racing (a sponsor here) about this - he said it is normal for the piston to "weep a little" - he disassembles the shock and cleans up a few small casting flaws to avoid any issues. A piston is actually more reliable than a bladder and is what is used in more expensive shocks - it is more expensive to build this way... The bladder is easier for the novice to bleed and has no other advantage - it doesn't change the shock action.
jeff