Put the green stuff in my Nissan truck about 6 weeks ago. Engine holds 3.5 with the filter so there isnt much in there to begin with. I have always changed the oil religously with Castrol GTX, Syntec Blend, Mobil DC GF-4, and Havoline oils. The engine appears spotless (a feat at 112k with that sump) on top of the head and such to boot. Well the GC has been in for like 1500 miles and is DARK. I do not mean black but a very dark amber on the stick. I assume this is the cleaning properties of this oil coming into play. Anyone else had such an experience? I have not had another oil do such a thing so I was just wondering.