I know what you mean about BMW and the coolant gauge. @fmorelli
, they have been buffering the coolant temp starting with the e36 cars. I've been banging my head on a wall ever since the e36 days and there is no temp gauge in the e90s, wtf...
I wouldn't worry about oil pressure with the added oil cooler. The cooler is the last part of the system so it wont affect oil pressure in systems that come before it enough to care.
Oil temp is measured from the oil level sensor. It is actually a multiple function sensor of level, oil condition, and temp. From the BMW TIS:
"The oil condition sensor consists of two cylinder capacitors arranged one above the other. The engine oil quality is measured by the lower, smaller capacitor. Two metal tubes are arranged one inside the other as capacitor electrodes. The engine oil is located between the electrodes as an electrical non-conductor (insulator). The electrical material property of the engine oil changes with increasing wear and dissipation of the fuel additive. The changed electrical material properties of the engine oil change the capacitance of the capacitor. This capacitance value is processed in the integrated electronic evaluation unit into a digital signal. The digital sensor signal is passed on as an indication of the engine oil quality to the digital motor electronics (DME). This actual value is processed in the DME for calculating the next engine oil service. The oil level is determined in the upper part of the sensor. This part of the sensor is located at the height of the oil level in the oil pan. As the oil level falls, the capacitance of the capacitor falls accordingly. This capacitance value is processed by the electronic evaluation unit into a digital signal and also sent to the DME. A temperature sensor has been fitted to the electronic evaluation unit to measure the engine oil temperature. The oil level, engine oil temperature and engine oil quality are measured continuously from ignition ON."
Back to the oil temp... I know the mesh has a large percentage of open area but I have seen it over and over cause air flow problems. Same with the louvers on the back. The turbulent air of the tire well doesn't help letting air escape either. To do it correctly and to truly know its right requires CFD which is out of the realm. Again if it were me I would put a fan on your cooler. This guarantees airflow through the cooler. I tell all my customers that road race, you need "belts and suspenders". A fan ensures you are covered in all scenarios such as drafting, hot days, grass build up from going off track, etc.
I haven't used it but give what @fmorelli
suggested a try. Simple and plugs into your OBDII port: "As a cheap solution, a bluetooth dongle and an android running the Torque app might help out (and data logging comes with it)." However on another note, as far as gauges go... A long time ago I concluded that there is nothing that beats a real data acquisition system. You get real data right in front of your face as you drive with right in front of your face alarms coupled with telemetry data via GPS. You simply can't get that with OBDII logging or analog gauges.
My conclusion and opinion is still the same, I would go oil cooler fan and see what happens. $100 for a fan and simple install before going into anything else way more expensive and complicated.
I really hope this helps!