[sperry]

Well, the 1st number isn't as important as the 2nd IIRC, at least in our application. The first nubmer is the cold weight of the oil. The second is the hot weight. The higher the number, the thicker the oil, the less prone to breaking down it is, as well as the more power it will rob from the motor.

For example, running 15w50 in the winter in your daily driver is probably a bad idea, since it will make the motor work extra hard pushing around all that thick oil. However, on a hot friggen day, when you're racing the motor, a 0w30 might be too light and could break-down reducing lubrication/protection.

At least that's the reasoning behind Mobil 1 selling their 15w50 as "for High Performance Cars".

One thing to note: Subaru's seem to have pretty good cooling systems, so even under extreme loads (racing on a hot day) the motor tends to stay around its normal temps, so the heavier oil probably isn't doing anything but lowering horsepower. Also, the difference between 0w30, 5w30, 10w30, and 15w50 probably isn't that much under normal operating temperatures. The 50 might be a bit thicker, but all the Xw30's are going to be the same at 180deg-220deg.

Anyway, that's my take on it... I certainly don't consider myself an expert on this topic.