I suspect some of the info on gearbox life and 4 vs 5 speed came from this on Duane Ausherman's web site. These comments are attributed to
Rick weber. http://w6rec.com/duane/bmw/trans/index.htm
"The secret to long tranny life is using synthetic gear lube, Spectro is the one I've used and had the most experience with. Usually 50k to 75k seemed average life span for the bearings getting regular oil changes with petro based lubes. You can at least double that with synthetic. I had one customer who had used nothing but synthetic and wanted me to inspect his gearbox that had 135k miles on it, and never had been opened. I swear it looked like brand new inside, no gray sludge, no metal on the drain plug. The 4 speed gear boxes seem to go longer than the 5 speeds, so I'd think the 4 speed could have a longer interval"