Ghengis, it looks like you may now have a interesting job for your retirement years, compiling the definitive book of Oscar Schrenks life and achievements - as it appears his work has never been fully documented anywhere, nor has he been given the recognition he deserves for his substantial contributions to the aviation engineering knowledge base.
He's not alone - there are plenty of pioneers and leaders in many industries whose work has gone virtually unnoticed and unlauded - even though the results of that work are enjoyed by many, without a thought for the researcher who discovered the principles utilised.
It's possible, that because Schrenk was German, and always saw himself as German (despite his substantial amount of work in the U.S.), there may be records in Germany of Shrenks life, that have never been translated into English.
As a result these are often overlooked by researchers in English-speaking countries. It's also possible that many German records relating to Schrenks life were obliterated during WW2 bombing.
Many records have yet to be put up for easy online access, and lie in dusty shelves of institutions and museums and Govt record depts - which records require actual legwork, to unearth.