Dairy causing more mucus is a myth, in general populations anyway. Now a dairy allergy causing more mucus is not a myth...some people have just a slight allergy that they dont notice anything but a little bit more mucus. Other have a little more allergy to where it causes stomach problems as well, and of course you have those that are severily allergic and can not consume even the tiniest ammount or it can cause them serious problems.
Milk can cause a feeling of thicker mucus, but does not cause it. Its because milk coats the throat, mucus thats around the throat and make that harder to cough up....but it doesnt cause all the mucus in the lungs to be thicker or more abundant.
I will say this dairy products react in ever person differently...so for one person it may make them feel worse and may make their mucus thicker while another person has no problem with it whatso ever. The question is really this are you willing to cut out dairy products...does dairy products make you feel any worse or any better. This is one of those topics even though it may seem like an easy question it really isnt. We can tell you our experiences with it, you can take what you learn and try to cut out dairy...if cutting dairy helps you then by all means cut dairy. One warning though if you have trouble gaining weight and want to try and cut dairy, find a good caloric substitute or dont cut out all dairy all together. Dairy is a good and healthy source of protiens and calories that some of us desperatly need. I say if you want to try this cut out milk first and reduce the intake of cheese...but dont completly have it all gone, that is if you need help gaining weight...if you dont have a weight problem cut it all out if you like. Also some people have a problem just cutting things out completly they tend to "cheat" here and there and then bam what they cut out is completly back. So allow your self one dairy item every now and then..like once a week...it should help cut the craving for whatever it is your craving.