I’m French and I can’t recall ever using (or heard someone using) “un”, regardless of the beer’s brand. I really can’t think of a brand where it’s “un”… And I know quite a lot of brands!

Since beer (“bière”) is a feminine word, always use “une”.

The general rule is that a brand take the gender of the generic name of its product. Because une bière is feminine, we can expect the vast majority of beer brands to be so too. This is true even if the brand name is formed around a masculine noun: for example une Piedbœuf (lit. foot-of-bull), despite pied being masculine.

However, there can be deviations from this pattern, of which we’ve (thanks Laure) identified two so far. These tend to be local usages, that have to be learned by the uninitiated, since they go against the habitual tendency of the language, and neither of our examples are masculine for every native French-speaker. But of course, there is a certain social cachet in knowing that a luxury product belongs to an unexpected noun class.

Orval, one of the Trappist beers, is masculine.

The traditional explanation is that, since the name is a compound that translate to Gold Valley, it takes the gender of its second element, like any French compound. Un val is masculine, thus un Orval is too. (But compare with Piedbœuf above!)

The locals of Gaume (the region around the brewery) reportedly go so far as to pluralize it on the model of “val” too: “Patron, deux Orvaux, s’i vos plaît bèn!”.

Any native speaker who’s unfamiliar with the brew is liable to say “une Orval fraîche”, “deux Orvals fraîches” though, on the model of every other beer name.

Also from Belgium, there’s a particular make of beer that originated from around Brussels called Lambic that is masculine. That too is unusual: we say une Trappiste, une Pils, une Gueuze, une Stout, and une Criek but un Lambic.

This unexpected gender is harder to explain. One possibility is that lambic is a truncation of alambic (alembic), a masculine word, but I haven’t managed to trace that etymology back to a scientific publication, only beer literature.

This masculine gender spills over to specific brands of Lambic beers. One can thus find customers in Brussels’ bars ordering “un Faro Lindemans”.

In closing, and as an illustration of the variability existing in the usage of the speakers, here is a quote from the Brussels daily Le Soir of the 18th of October 1988 :

Et à Bruxelles, cela ne devrait pas vous étonner, c’étaient la gueuze, le faro et la lambic qui avaient les préférences du public, n’en déplaise à Charles Baudelaire lui-même qui qualifia ces bières de «deux fois bues».

I don’t know what the rules are for that. But for my experience, I always order “Une ZzZ”. Then eventually the bar tender ask me : “en pinte ou en demi ?”. If I want to ask for a demi I always go for “Une ZzZ, en demi”.

I think in this case it’s because the name of the beer has mixed with the object itself. Like “Passe moi le scotch”, in fact is a abuse to say “passe moi le rubant adésif de la marque scotch”. And since the object itself is feminine, we say “Une”.