I live in Minnesota, and it has been 20F or colder recently. I have been paying close attention to the "yellow oil sludge" issue and changing out the oil more frequently. Recently the car has not been coming up to temperature. The water temp gauge reads less than 1/4. Other than this, the car seems to be running fine. Any suggestions are appreciated.

sounds like a blockage problem . feel all the water pipes when the vehicle is at its hottest . possible cause water stat

I had experienced a similar problem on my e39 sometime ago. T-stat stays open even when engine temp is cold. Replaced t-stat and the problem went away. Good luck,

bad t-stat, replace it as stated