Bernd
I use Rocol RTD on mine, it's a fairly well known stuff for tapping & cutting. Certainly makes a difference screwing stainless, ( usually EN58, your '303 I think, not sure ).
But were you using machine reamers ? I have a 1/4" with straight flutes, this seems to be OK. I get on OK with FCMS ( Free Cutting Mild Steel ), but with brass or especially ally I get score marks or chatter marks, even with the same reamer. The reamers are mostly Intal or Dormer, so they aren't Asian tools, nor are they worn, some are as new. I usually ream at about 300 rpm, feed, I guess, at about 1" per 2 secs.
I have been using a lathe since 1971, always had problems with brass & ally. I've just about given up. I can't ream. I need to live with it.
Dave BC