Originally Posted by Irelands child
With OEM fittings which are steel, you get into double flares. With double flares, you get into correct sealing as well as strength which is a plus with using the steel fittings. I'm not sure that you can even purchase a brass fitting that will use that double flare. My recommendation is that you use steel fittings - and yes they will rust, but don't over tighten, don't use any loc-tite and use the correct wrenches and you should not have a future problem.
Uh, no. The single/double flare pertains to what you do with the end of the tube. It has nothing to do with the fitting. ALL 45 degree inverted flare fittings, whether brass or steel, are dimensionally equivalent. Both work. The steel are stronger, the brass don't rust and are strong enough. Your call. I typically use steel for the flare nuts that go on the tube, but the fittings are typically brass. Haven't had a problem in 40 years of doing this.