I didn't think bienchen said "when" was grammatically incorrect, but more that it was less suitable in certain cases ... the thread is a bit long, though, may have missed something!
When and if are often more or less interchangeable with zero conditional
If I see the Queen, I am happy.
When I see the Queen, I am happy
This charge does not apply when/if you are club members
(Technically there may be a difference, but the meaning is the same: people who are club member don't have to pay.)
They are not interchangeable with first conditional sentences, http://web2.uvcs.uvic.ca/elc/studyzone/330/gr...
or at least I can't think of a first conditional sentence off-hand where they are interchangeable:
a If I see the Queen I'll be thrilled. (I might see her.)
b When I see the Queen I'll be thrilled. (I am definitely going to see her.)
c If you google for this term, you will find ... (I think you might google)
d When you google for this term, you will find ... (I'm sure you are going to google)
Here, c and d are closer in meaning, as it is not such a big deal whether or not you google, what is important is what you will find: I'm telling you it is true, whether you google or not! With a and b it makes much more of a difference: in a) I'm hopeful but unsure, in b) I'm excited and looking forward to something.