It could be caused by not having a thermostat keeping it at operating temp.

Not having the exhaust preheat system hooked up causes engine to take 10 minutes to heat up instead of 2 minutes. Throw in
stop and go driving.

In the frozen north part of the radiator has to be blocked to get engine to operating temp at all or in a timely manner.

The above means water stays in the oil instead of boiling away. This is major cause of sludge.

Also not doing oil changes will cause sludge.