I did my PhD on topic X, and my PhD advisor is an expert on X and on Y. For a problem I have come across in my postdoc, I think that Y would be a good approach. There is no one I am currently working with that I could ask about Y, so I would like to send my PhD advisor an email asking for literature recommendations on topic Y. Is this customary? What would be the most polite way to ask? What is the best way to go about this?
Similar to business, one important key to be successful in Academia is having contacts and collaborations. Your relationship with your former advisor didn't finish on the day of your PhD examination. You should be one of his close colleagues in the domain and this applies to you as well. Otherwise, where to find collaborators for project proposals and joint papers, etc?
Conferences, symposiums and workshops are made to meet people, discuss with them and open new collaborations. If this is the case with people we meet for a week, how should we deal with people we worked with for years?
In my opinion, if you work in the same domain as your former advisor (or any other colleague) and you don't ask for his/her advice, it is strange academic behaviour.