Keith
Actually it is possible that changing the motor oil could be the cause of the poor idle. It is very rare this happens but it does, although I've never seen it on a Subaru so far. In some cases where the carb is in very poor shape or the float level is wrong gas can end up in the oil pan. Setting the carb idle adjustment while the gas is in the oil makes a difference because the gas fumes from the crankcase are being sucked into the carb and can cause a faster idle which is then compensated for by turning the idle down on the carb. As soon as new oil is put in the motor the extra fumes are gone and the idle drops. Like I said, rare but it does happen. Barry