There's no question that shiny AN fittings look great in the engine bay, but as off-roaders we need to put function first. Fortunately, there's not one, but several functional reasons to make the switch.
A bit of history is in order before we proceed. AN fittings had their genesis during World War II. AN stands for Air Corps/Navy.
Some nomenclature needs to be noted, too. AN sizes are a bit odd this way. In this story, we're using -6 fittings and hose. The size -6 is spoken as "dash six." The "six" is the inside diameter of the hose measured in sixteenths of an inch. As such, -6 fittings are 6/16-inch, which is the same as 3/8-inch.
AN fittings use an angled interface at the couplings. The angle is 37 degrees. This is important because SAE fittings have a 45-degree angle. Some adapters exist that have a 37-degree and a 45-degree surface on the same fitting, but unless you've got one of these adapters, you can't mix AN and SAE fittings on the same coupling.
Now that the background info is out of the way, what are functional reasons to use AN fittings? They're easy to disconnect and reconnect. They're rugged. The hose ends are re-usable. Hoses are available in many styles and strengths to perform a variety of plumbing tasks on your truck.
A few months ago, we installed a Pareto Point Industries Topdog V oil filtration system on our '04 4Runner. The Topdog V comes with push-on fittings and hose that work perfectly well, so that's what we used for our initial installation.
Why ditch the push-on fittings for AN plumbing? Push-on hose can't be removed from the barbed fittings without cutting it off. At that point, the hose is likely too short and you'll need new hoses. Since AN fittings can easily be removed and re-connected, we'll be able to get the hoses out of the way if needed. The original push-on hoses were 1/4-inch I.D. We went up to 3/8-inch or -6. The larger hoses will hold a bit more oil and flow more freely. In the same breath, it bears mentioning that the super-fine filter media in the Amsoil EaBP-90 bypass filter is the biggest limiting factor to oil flow, so we'd have been fine going with -4 AN hose and fittings. Overkill's my favorite method, so -6 it was.