Vehicle is 2005 Chevy Trail Blazer with the former, in great shape 4.2 L I-6 cylinder engine with 7 qrt. sump : I use a variety of synthetic 5W30 oils (mainly PP and M1) for an OCI indicated by the GM oil monitor with good results - I could even probably go longer with the 7 qrt. sump and same synthetic oil ... My potential concern is I am using Castrol Edge 5W30 synthetic for the first time - after only a few thousand miles into the OCI the oil is almost black (and that is with a Fram Ultra oil filter) . Is this a normal occurrence with Castrol Synthetic oils ? This vehicle is run with all highway miles and I don't see oil this dark until the end of a normal OCI using PP , M1 or even Super Tech SN rated synthetic oils ... Your thoughts ?