This is how I checked mine:
After you warm the car, pull it into the garage and let it sit and idle.
Stick a cooking thermometer in the oil tank (suspend it).
Periodically feel the oil cooler until it starts to warm.
Note the temperature.
Trash the old temp gauge and install a numeric one.
Worked for me.
What I found was that at sustained highway speeds in cold weather the engine does not heat up enough to open the thermostat. This may be a 930 specific issue due to the fuel cooling and lower C/R of those engines. One thing that helped was to block off my giant tail, restricting flow to only go through the intercooler.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8