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DriverX5: Assume you mean with an automatic. If it is with a manual, definitely move it to neutral to save the clutch throw-out bearing.
I can't imagine that there would be any measurable fuel saving by moving an automatic into neutral at a traffic light. If you cared about fuel economy that much, you would turn your engine off if you were going to idle for more than about 20 seconds. No, I am not advising you do that, since you wouldn't be able to move if you saw someone bearing down on you, but it is the principle exploited by hybrids.
When you are stopped at a traffic light in drive the gears in an automatic aren't moving, so there is no gear wear. The gears are attached to the output shaft. The disconnect is at the torque converter, after the engine and before the transmission. What you are doing is generating a small amount of heat due to the slippage in the torque converter. That heat is dissipated by the transmission oil cooler.
With heavy equipment, powershift transmission life can be correlated to the number of lifetime shifts, more than the number of hours of operation. I don't have similar studies on automobiles, but there are reasons to believe that there would be similar correlations. It is only when you shift that the automatic transmission clutch packs and bands wear as they engage. I would trade the wear and tear on the trans fluid for the wear and tear on the clutch packs any day.
Jeff
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2007 X3 3.0si, 6 MT, Premium, White
Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue
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