My coworker has a 2012 Buick Enclave with approximately 142k miles. She had to have new cam sensors installed. While it was in, the dealership noticed that it was severely sludged in the timing chain. They replaced the timing chain and changed the oil while they had it in. She did oil changes based on the Oil Life Monitor. Dealership told her to change the oil every 4k miles and ignore the OLM or switch to full synthetic and go about 6k miles. I have a 2016 Dodge Grand Caravan. I use full synthetic and go by the OLM. Based upon the OLM, I usually go about 10k miles before changing oil. I'm concerned about the possibility of sludge. Recommendations?