I'm not saying problem either, my cars nothing like a C5 and I don't know much about the C5.
Coolant being slow to warm up is usually a faulty thermostat, whether yours is slow or normal I don't know.
Hi. On my C5X7 2.0 HDI from new it was this way. First oil temp rised for 4 stripes than coolant started to catch up and bolth ended on 7 stripes. If pushing hard oil raised to 8 stripes, coolant always sitting on 7. Last few weeks takes ages to heat up engine. Oil comes to 1 or 2 and tjen coolant rises faster than oil. After stabikisation coolant falls from 7 to 6 and oil sits on 7. Water thermostat works, at least hose to radiator is cold till gauge reaches 6 stripes.... What could be the problem of oil temp rising so slow all the suden? Any clue, anyone?