I suspect that quite a few rear wheel drive standard shifts fall into the same category.
When the transmission is being driven, the input shaft is constant mesh with the cluster gear, and the cluster gear is constant meshed with the individual constant mesh gears. All of this is spinning when being driven, or idling in neutral with the clutch released, and fluid is being sprayed everywhere by the gears.
When being towed, the only thing in the transmission that is spinning is the output shaft, no gears to throw up lube and keep the bearings and bushings lubed. I was looking at a nice BMW Z3 that was for sale nearby. Thought it would be a neat toad for my Winnebago View. Then I discovered that no one made baseplates for them, and went to the Z3 forum and did some searches and found out why. Fluid level is fairly low and thus the mainshaft will quickly run dry when being towed.
I have an old '67 Ford Falcon with a three speed. For years my Dad and I both had problems with the rear mainshaft bearing failing, and the slip yoke bushing wearing out. Finally I concluded the fill plug was engineered too low and not enough fluid was making its way to the bearing and bushing, even in normal driving. I overfilled the transmission thru the top cover and installed it, putting it nose down when I removed the dummy yoke and installed the drive shaft, and have never had a rear bearing or slip yoke bushing failure since. In the course of a clutch replacement I replaced the fill plug with a iron pipe street ell which allowed me to fill to the higher level thru the fill plug.
Charles
'03 Ram 2500 CTD, 5.9HO six speed, PacBrake Exh Brake, std cab, long bed, Leer top and 2008 Bigfoot 25B21RB.. previously (both gone) 2008 Thor/Dutchman Freedom Spirit 180 & 2007 Winnebago View 23H Motorhome.