According to the FAQ, Subaru wants you to use time as a factor. Its not just the possible accumulation of moisture that's an issue. This time of year (cold, winter temps) your car spends more time in "open loop". This means the ECU has the engine in warm up mode and is pretty much ignoring the O2 sensor to control fuel mixture. Its like a choke on a carb engine. A richer fuel mixture is not only ineffecient, but it also results in unburned fuel. That unburned fuel is washing down the cylinder walls and contaminating the oil. If you don't change the oil on a regular basis (three months), you get some pretty nasty stuff in the crankcase. This is part of the reason that your oil gets black when its dirty (the other is blowby).