I just took the plunge and put AMSOIL 5W/30 synthetic with a Wix filter into my 2001 Pathfinder. I checked the oil level the same day and found it to be slightly dark.....they left some of the residual oil in the engine. How much will this rob me of the benefits of the AMSOIL? Will I have to change the oil/filter sooner because of this? Your input is much appreciated.