On another forum I read about somebody losing a Chrysler 2.7 engine because the engine sludged up, the hydraulic timing chain tensioner did not work properly, the chain jumped and destroyed the engine. Considering all the problems oil sludging causes, why not avoid it with shorter oil change intervals? I used to go with 3k mi OCI, I now use 4k mi OCI (on dino oil). This is with 90% city driving. The OLM on the Sierra comes on about every 5k mi, so I am changing the oil a bit more frequently than the OLM recommends. I like to get the oil nice and hot before I change the oil and I let it drain for at least ten minutes. I have never had any oil sludge in any of my vehicles. I have never had to use AutoRx or any engine rinses. Seems to me the extra cost of more frequent oil changes is well worth it to avoid the sludge.