If a engine is clean and oil been changed regually then its always a good idear to use engine flush, I do. I change my oil every 2500 miles mybe i am a bit fussey .

If i was you i would change it every 3K , 6K is a bit long in a cossie, exspecally if you drive it how it should be driven, Never use engine flush in a engine full of black sluge as it ends up in the sump and blocks the oil strainer,

Dave