"The reason the Boxer engine
"The reason the Boxer engine seems to be more prone to have this problem is because of the engine design.
Subaru uses the Boxer engine which has a horizontal design and pistons lay flat in the cylinder. When the engine is turned off, the fluids tend to pool next to the head gaskets instead of draining, and over time acids in the fluids eat away at the seals. The gaskets deteriorate and oil and coolant fluids start leaking."
This whole paragraph should be removed because it's utter bullshit. You had it right in the previous paragraph. The problem lies in the gasket itself. Subaru used a graphite coated single layer steel gasket in the EJ25D (most noteably) as well as the EJ253. These were the only engines that had widespread headgasket failure and it wasn't because the "fluids pooled around the gasket. What? You think when you shut off a V, I, or W type engine the coolant drains out of the head and into the block? It's a filled system with no air in it. Every headgasket in every piston engine is in constant contact with coolant no matter if it's running or not. There hould be no air in the system. This "theory" is the dumbest myth on the internet. It doesn't even make sense.