I believe sludge happens when you don't do in between oil changes. I had a previous post about this. I even had pics to back up what I was explaining. Either way, my mechanic had this one customer, he brought his Bmw 540 03 in because his engine was not running right. At the end of this problem, to make a long story short. My mechanic opened the engine and to see all this sludge, grime, basically SH&T! in the engine. This guy did only every 10 to 15k oil changes. My point here is if you do in between oil, and the X is not running with oil past 5k to7k then there shouldn't be any sludge in the engine. But like I said before. I flush my engine every other oil change or even every 3rd, whatever it really doesn't matter much, this stuff will not harm anything.