[quote=Gebo]I think the Castrol was cleaning the engine. It may be that if you did your next change with Castrol that it wouldn't turn dark brown so quickly. I would do an experiment the next OC and use the Castrol like you first did.
^^this is as close to an answer as you might get^^
Both good motor oils and have not pushed either far … No worries.
It's really weird how certain oils react differently to different engines and driving styles. What works for one doesn't work for the other.
I've just recently used both the Castrol Edge and Valvoline Advanced. The Castrol stayed amber much longer than the Valvoline Advanced, which turned black in under 2K. That has me to believe the Advanced has more cleaning power than Castrol Edge.
What I witnessed was done in under 11k total engine miles.... right after the factory-fill was pulled at 3K.
So all theories get blown out of the water at our BITOG residence....lol