I am IT Director of a Company and I'm regularly invited by Italian universities to do lectures on Innovation or Information Systems, usually 5/6 lectures per year. Can I claim University Guest Lecturer title in my CV?
I pretty strongly suggest that you don't claim a title of any kind unless it is formally conferred. That is, if you want your CV to be credible.
But there is a difference between "University Guest Lecturer" and "university guest lecturer". The first seems to be a title, but the latter is just descriptive. You can certainly describe what you do in a CV without making any claim to hold a position that the university hasn't actually conferred.
Caution is best, I think, in all such situations.
A "lecturer" is usually considered an academic title. So if you are just giving an invited talk, that would usually be considered a seminar; if you are invited to teach an occasional 6-week long course, then maybe it could be justified to call yourself a "guest lecturer" for that period.
Whether or not you "can" do this is somewhat subjective. You do occasionally give lectures as a guest, so you are not technically lying. But I would suspect that if someone called you on this and you had to explain yourself, it would not look especially flattering.
Are your lecturing with a formal contract, or is it just a volunteer activity? Does your name appear on the course syllabus and on the university website?
If yes, that would fall into the case of "professore a contratto" (even if just for a part of a course), and I would endorse the translation as as guest lectures.
Otherwise, it would seem like an abuse of the title.