My 750 STOL/3300 Jabiru's AN-8 oil cooler lines have stainless steel female fittings and my oil cooler's fittings are male aluminum.  I've seen advice to not mix steel and aluminum fittings and others have the opinion it's not a problem.  Recently, after installing a new oil cooler with my new Gen 4 engine, one of the connections had a very slight leak.  Both the oil line fittings and the cooler fittings were being re-used from the previous installation.

I stumbled across these DEL fitting seals at Aircraft Spruce:

I bought some (they're cheap compared to new fittings!) and installed one on the leaking connection.  It seats on the male fitting and basically creates a new sealing surface, covering any slight scratches or other minor defects that prevent a good seal:

You can get them in copper or aluminum - I got the aluminum ones but I think either work equally well.  They are very thin and very delicate, but ACS packed them inside a protective cardboard tube and they arrived in perfect condition.

The seal worked great and cured the leak.  I think they're intended for one-time use, so I plan to keep a couple of A-8's and AN-6's on-hand in case a leak ever develops in the future in the oil or fuel lines.

I may be the last person to know about these seals, but thought it might be worth it to post about it in case someone else doesn't know about them and it might save them the expense of buying a new fitting.  FWIW, Spruce says they're approved by NASA, SAE, and the US military.

John

N750A