I bought my 05 Ford Crown Victoria with 20K miles on it at the end of 05. It now has 75K miles. Up until 50K miles I was using 5w20 Motorcraft Blend. Then I switched over to 5w20 Motorcraft Full Synthetic. I change out every 5K miles. I notice though at 5K miles the Full Synthetic is allot more dirtier then the Blend. The Blend would be a median brown where the Full Synthetic is a very dark brown though not black. Is that anything to worry about or is it darker because it cleans the engine better? BTW I never have to add between changes on the Blend or Full Synthetic. I am hoping to keep the car for at least 200K miles or more like I did with my last car which was a 98 Crown Vic. The 98 used 5w30 Regular Dino Oil.