Please don't mind "lore diehards" - every mentor in World of Darkness has their own agenda and their own world view. Beckett wasn't fully objective, neither was Smiling Jack, and your Gangrel doesn't have to be, either. The view on Rudi, Xaviar etc. can be inspired by the book, but doesn't have to be 1:1.
In general, yes - best to base heavy lore bits only on the new book, but again, subjective narrators are totally a thing and we're cool with that!