I went to check what GvFC says in the documentation they send with their pens. Here is what they say under "Hints on care:", and I quote (a photography can be provided, if necessary):
"For maximum convenience we suggest you use only water-based inks such as Graf von Faber-Castell ink." (emphasis added by me)
As one can see, much less threatening than the language used in their correspondence. In fact, there is no mention of voiding the warranty and it is only a suggestion. In fact, they don't say you can use only GvFC inks, but suggest that any water-based one is good enough.
I do like GvFC pens, particularly the wood/metal ones. That does not mean that I have to consider their customer service top notch. In my opinion, it is not. GvFC pens are very expensive: a Classic Anello costs about as much as a M800 or a 146 here in the US, but my experience is that their customer service is not on par with the customer service provided by those companies (not that theirs is perfect either) -- and you can make your own mind about which is the better pen (for me, it is not the GvFC)
I fully believe that Mr. Binder is correct that some inks can damage the feeds. But feeds are inexpensive and the same is true for every other brand, yet they do not make such threats and offer other services, such as nib exchange.
Finally, the GvFC pens are good and have good design (IMO). But in my experience there are other pens that, for the same price, are better.
My experience with their pens is the following: my wife and I have 4 GvFC (a Classic Grenadilla, an Intuition Terracota, a Snakewood LE and an Elemento LE), all of them have good, smooth, but not particularly remarkable nibs. But all of them are also very fussy about the ink they take and have a tendency to dry out in the middle of writing with a converter full of ink, and that is even using GvFC ink (which I happen to like very much).
All that said, I'm still in the market for an Intuition Wood Pernambuco or Ebony, and I keep a close eye on their LE pens for something that catches my fancy. Why? Well, I'm a sucker!