Well, a simple fix I have done....did it just today too.  2 qts ATF and 2 qts diesel.  Run it for 10 mins til its good and warm.  Dump it and put new oil and filter in.

Here are a few details to think about.  

As mentioned previously, the special o-ring at the cam case is to be the Subaru o-ring. During my adventures at PnP and removing cam towers, I am not surprised to find  brittle, deformed and crumbling o-rings. Bad idea to use regular o-rings.

'Oil relief tubes' at the cam tower. There are relief valves. The springs get tired. Inspect the springs. Stretch the old ones while you order new 'relief springs' from Subaru. Never use silicone on the cam towers.

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I love it when that little o-ring comes out looking like a heart, for love you know.. its what makes a subaru a ticking subaru.

 

It would make sense also that a failed relief spring would allow HLA's to tick, it would be like taking your finger off a water hose. It would be nice to add a pressure sensor to the HLA galley or cam tower just behind the relief valve. Both are fairly accessible for tapping.