[Bobcat]

You asked if steel fittings would work better . We had the same problem with them on hydraulic systems ( aluminum manifold blocks and steel NPT fittings ) .... after being torqued down and going through many heat cycles , they "froze " in the blocks . We used to take a hammer , put a piece of steel pipe over the fitting and on the wrench flats and give it several good whacks to break it loose .... but these were 2 " fittings . With aluminum , Never Seize seems to work ok , but the fitting will still be difficult to remove after a long period .
Given the option , I would always use SAE straight thread " O" ring fittings .
I hate to use NPT on anything , but sometimes there is no choice .