I recently had a radiator problem... in the end I replaced the radiator and used BMW coolant...
But in the mean time of tracking down the leak, I just used some better stuff from auto zone....
I say find your leak with the cheap stuff, (so say its only in there for a couple of weeks) then flush, fill with bmw coolant, and feel good about it for the next two years
I guess all those coolant hoses take a dive after a few years... so I'd start by checking with those.... Another thing I did was to use the garden hose to spray off affected areas, getting the parts nice and clean.... that might make it easier to clearly identify where the leak is comming from.
Also check www.bmwe34.net I'm not sure if there are any exact fixes for your problem, but in general its a great wealth of knowledge! and give www.bmaparts.com a look for purchases.
another quick point, for the m60s in the 530s, (maybe someone can back me up here) but I'm pretty sure that you don't/can't blead the coolant system. but I did put some coolant in and squeeze the upper hose to help move some coolant through.