serah, from what I know it is. The milk changes for the newborn, rather than staying right for the older baby.
I also gather that, the more babies you feed, the better supply you have. Mine has certainly gone from somewhat-sparse-seeming to quite adequate. (Granted, DS2 had a much much better start than DS1, which probably helps.)
Oh, hercules, I have a bone to pick: the milk isn't the same at 6 months as it is at 8 weeks. The milk changes constantly to suit the baby's growing needs, and to provide antibodies to whatever viruses and bacteria are in the baby's environment. Every time a bf mother kisses her baby, she picks up the bacteria and viruses that might attack him, so she can make antibodies for them. BF is such an incredible thing. (hercules, I know you already know all this, and I know what you meant ...)