BMW Repair: BMW coolent level?, dexcool coolant, radiator cap

Question
Hi,
I own a 525i (E39) BMW and last week while driving I noticed the temperature gauge was half way between the halfway mark and the red zone.  I was very worried and immediately went home and put some new coolent in.  Since then, the temperature had not gone beyond half-way.

Some more background on the car, I bought this car secondhand from a dealer.  The car had done 100,000km.  It still drives well, not much is wrong...so far.  And below are some of my questions, which I hope you could help me with:

1).  Where should the temp. gauge usually be?  Less than half-way, at half-way or a little above.

2).  If the temperature is always at half-way is that ok?

3).  I topped up with tectaloy 90Plus (ready to use stuff) is this ok?  If not, what should be the coolent to use for this type of cars?

4).  Will it matter if the existing coolent is different from the Tectaloy that I used to top up?

5).  I think I over-filled it as the top of the floater is now about 3 centimeters above the level of the cap when I removed it, is that going to  be  a problem?

6).  I am thinking of getting my transmission oil replaced also, is this a good idea as BMW states in their manual that the transmission oil is lifetime...?

Cheers and thanks,

Answer
khnah, Temperature should be Halfway when the engine is up to operating temperature and no more than just a little bit over half. BMW coolant is supposed to be organic and therefore special. As long as you use a conventional ethelene glychol coolant you should be okay. Don't use General Motors Dexcool coolant. BMW coolant is usually $25.00 US/Gallon and is concentrate so it should be cut in half with water to make two gallons. I'm not familiar with Tectaloy so I don't know what it will mix with. Don't worry about over filling. The overpressure valve in the radiator cap should allow extra coolant to escape. I would indeed replace trans. fluid and filter. Draining the pan will only remove about a quarter of the fluid in the trans. Most of the fluid is in the torque converter. So you may need to do a drain and fill several times. Only use BMW approved fluid and check fluid level between 30-50 degrees celcius. Since the trans warranty is not lifetime neither should the fluid remain lifetime. (just my opinion)
-Evan