Many people do use AL. So far no one has reported a failure. I used steel. You can get steel from a hydraulic hose shop locally. I'm careful to not buy the ones that say "China" on them. I think some Parker fittings now say "China" some do not. I found some -8 Weatherhead steel that did not say "China" and that's what I have on the aircraft right now.
You will find people say "All fittings steel firewall forward". Usually some AL finds its way in there.
Pacific Oil Cooler Services Inc. has posted some insightful information concerning aluminum vs. steel fittings in oil coolers. Bottom line is they don't recomment alluminum fittings for oil coolers.
FYI: Common Oil Cooler Woes, early detection and prevention
