Most manual transmissions are splash lubricated, regardless of the type of lubricant used.
In many (if not most) cases, the lubricant is splashed by the cluster gear, which ONLY turns when the input shaft is turning. The input shaft is splined to the clutch disc, which only turns when the engine is running.
So, if the engine is not running, the transmission is not being lubricated. There are at least two bearing assemblies in most transmission mainshafts; a needle bearing in the middle of the shaft, and a ball bearing on the output shaft, along with a seal. With no lubrication, these bearing WILL eventually fail, as will the seal. How far you can tow the vehicle without damage is unknown.
Personally, I would not take the chance. I would disconnect the drive shaft at the differential yoke, tape the universal joint caps so they don't fall off, batg the attaching parts, and tie the drive shaft up to the underbody very securely. I used to do that when I towed my old 4X2 Datsun diesel pickup. It took less than 5 minutes to remove or reinstall the u-joint.
