I would be either happy or scared if the GC was still amber after a period of time in my engine.
When this was mentioned, it was a vehicle that had run Mobil 1 it's whole life. There is a very good chance that there is no sludge in this engine at all - Therefore, all this oil has to do is clean up after itself and not the previous oils. GC has been talked up quite well around hear and shouldn't any deposits behind. In certain clean engines it is very possible that GC could still be amber after running for a while.
Now, on the other hand. If I had a car that ran dino it's whole life at 5000 miles OCI's I would be very concerned if the oil was not turning very dark after 3,000 miles. This would tell me that the additive package isn't working and all of that goo is still in my engine and not suspended in the oil.
FYI, one car I had ('84 Olds Cutlass 307 ci V8) maintained a clear oil after 3,000 miles. It was darker, but I could see through it while it was draining. This engine had over 100k miles on it. I had always run Castrol GTX 10w-30 in it. The previous owner, a retired gentleman, had the dealership do all the maintenance on the car. This car was in great shape with the exception of being waxed and polished too often - The paint was worn off at all of the 90 degree corners and the pinstripe was almost non-existant.