Castrol Syntec (5W-40)

[Perfectlap]

I recently changed my oil from Mobil 1 "factory fill" 0W-40 to Castrol Syntec 5W-40 after Advanced Auto had a special promotion of 5 quarts for $25 and a free K&N oil filter. I picked up 20 quarts to save some dough. I decided against Royal Purple at the last minute due to price and availability.
A local Mobil 1 oil change shop (actually no longer owned by Mobil 1 but everything else is the same) agreed to do the change for $30 and use my oil and filter even though they sell the same Castrol Syntec for much higher. We left the engine to drain for about an hour and 15. My 2000 S recently passed the 60K mile mark without any major issues. The only unscheduled repair has been a new throttle body and replacing an 02 sensor. That's it. This is not a daily driver and I change teh oil every 7K miles or so.

So my first post Mobil 1 impressions of CS: Something has definitely changed.
I've had m1 oil changes before and didn't really notice a difference. After a couple of days of the oil running I'm strongly inclined to say the engine runs much crisper and smoother after a CS oil change than a M1 oil change. It's most noticeable from a standing start. The revs seem to be "flawless" if I can put it that way. Almost as if the engine is not working as hard to get the car up to 70mph or so.
Obviously this is not a very scientific analysis. I had some concerns about introducing a new mixture into the engine while there was still some M1 in there. A friend also made a switch from a different oil to CS and soon noticed some strange looking oil residue near the oil cap that had a tan/greyish color about it.
The engine also sounds quieter when idling. I'm wondering if this more to do with changing the weight of the oil from M1 0-40 or the Boxster engine just liking Castrol's mix better. Going forward I'm going to change the oil every 5K miles to take advantage of the lower price for Castrol Syntec.