Double Dark Side! Diesel & 8P
From what I've been reading recently, oil starts to break down at around 130 degrees unless you have some decent oil. The OEM cooler is cooled off the engine coolant so the oil temp will climb in traffic as the coolant temp does with no air going through the radiator. I'm currently looking into fitting a 13 row oil cooler on top of my FMIC to keep temps down but I'm unsure how it will behave in traffic.
Out of interest where does your oil temp gauge take the reading from? I know that a reading from a sandwich plate between oil cooler and oil filter can give a good 5 degree Celsius more in temp compared to a reading from the sump.