Unfortunately today's thermostats are not the simple 5-min. job of dropping in a $10 metal device.
The thermostat costs a lot more now and is more involved to replace.
Seldom done in isolation, but as part of a complete cooling system overhaul.
Before diving in to change the thermostat, I'd eliminate a few other possibilities.
If the thermostat is stuck open, you should be noticing that the temp. gauge takes longer to reach the center position than it used to after a cold start (in any weather).
Surprising that once up to operating temp. the temp. gauge can go down to the blue, however high the hwy. speed is and whatever the air temp. - even if it really was 20 below, which it wasn't.
Maybe the switch for the engine cooling fan (electric) has gone bad and the fan is on continually.
Air pockets in the cooling system can produce strange symptoms.
Has the coolant be changed recently? (Should be every 4 years, with BMW blue coolant.)
Any chance some orange coolant was put in there at some point?
Did you definitely check the coolant level when the car was completely cold? Can be quite a difference.
The coolant level sensor in the expansion tank is a weak point according to the 3-series forums - it might be giving you a false reading.
Don't rule out the possibility of some localized overheating whatever the temp. gauge says.
At your mileage you shouldn't be expecting a thermostat failure - an expansion tank maybe.
When overheating is detected, there are apparently safeguards in BMWs including stopping flow of cabin heat to minimize any risk of scalding occupants.
AFTERTHOUGHT: I see the Bokchoys article above does confirm that a stuck thermostat can cause the engine to cool substantially when on the highway, so that is quite conclusive.
Last edited by Supercourse; 02-01-2013 at 12:42 PM.