Perhaps the high ester content of the Castrol dislodged something. I would change the filter and/or oil (again with Castrol) to be sure before condemning it.
I just switched to GC in my Passat(1.8T) and do not notice anything different. Everything sounds fine on startup. The previous oil was Lubro-Moly 5W-40 and I'm still using the Mann 719/30 oil filters. I can't comment on gas mileage, but I don't suspect there will be any difference.
I solved the cold start problem with German Castrol by adding 7 ounces of Valvoline Synpower Oil Treatment. The noise is completely gone now. At first I added a 1/4 quart of Mobil 1 15w-50 and it quieted the cold starts by about 75%, but after adding the Synpower Oil Treatment, cold starts are silky smooth. The car sat around for 4 days straight before starting it today, and the motor starts really quiet now. I have a feeling the copious amounts of moly and boron in the Synpower additive did the trick. The Mobil 1 15w-50 was just along for the ride at this point.
I'm happy that the Synpower additive that worked, because I was about to dump $30 worth of GC. I'll most likely move on to Mobil 1 0w-40 or possibly Synergyn 5w30 on the next oil change. The current brew of GC will stay in for 7500 miles and will see how it does.
I see your in NC,I'm in MD so our climate conditions aren't all so different.
Like I posted earlier I put Mobil 1 5W30 in my 00 GTI VR6 with no problems at all.My first engine start after I changed the oil(when the filter was pretty dry)resulted in absolutely no start up noise what so ever
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This morning I started it at 36.0F with no noises at all,just a smooth quick start
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Maybe that GC is too thick at temperatures above freezing
A little more food for thought: in my last car, a V-6 Camry, I got louder, rougher starts for about the first ~1000 miles or so on GC. Then it smoothed out very nicely. Why I have no idea.
Yesterday, I put GC into my new car, an Infiniti G35. Cold start this morning was purrrrr-fect -- no hint of start-up roughness at all. I'd heard that the VQ series V-6s like GC, and all of 24 hours into its first fill, it seems really good. UOA in a month or so.