Like I said, no surprise. Different engines have different sensitivities.
The Subaru engines (EA82 variety) are prone to lifter clatter due to very small oil ports in the lifters (technically "lash adjusters"). Also, they have pressure relief valves - one in each head - that regulate pressure up in the valve train area. Also, keep in mind that the oil pressure at idle on those engines drops pretty close to zero at normal operating temperatures.
Within a few days of suddenly switching over to synthetic, the lifters started clattering (the oil had been changed religiously using Castrol, which I consider a quality oil). I switched back to non-synthetic and, over a period of months, I was able to get rid of the clatter by using MMO in the oil and changing the oil and filter relatively frequently. My theory is that some crud got into the lifter ports or in the pressure relief valves, and had to work its way out. This scenario, with minor variations, was repeated by several others on Subaru forums.
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")