What are BN, V, M83248, PU series?: BN series (nitrile) is the cheapest type usually found in hydraulic and plumbing shops. Most divers use them on one-time use basis or on rental equipments. Salt water, UV, oxygen and ozone attack nitrile causing to fail. V series are FKM (Viton) which is far superior than nitrile. V will withstand against those elements which attack nitrile. NOX is ok with V series. M83248 is military grade FKM. PU is polyurethane. It's clear amber in color, and some divers prefer its "prettiness" and resistance to abrasion. I've seen some postings recommending other types of synthetic rubber for dive use, some on EPDM too. There are definitely other synthetic materials which can be used in scuba application, but probably not worth your while to investigate especially if you plan to experiment on yourself. :)
70 vs. 90: It's all about the hardness or durometer. 90 is harder than 70. 90 can take more pressure than 70, but 70 is softer and supple. So HP and tank necks use 90, and LP uses 70.