The problem with this question is that it has been answered lots of times by people who don't know what they're talking about, so I thought I'd ask here in hopes that somebody understands the question I'm asking and not the question they feel like answering.
Here are some points to consider:
1) I don't care if it's illegal to drive downhill with no gear engaged, that's not the question.
2) I am not concerned with fuel economy in regards to this question, but I will say a few things on that topic, because I know somebody's just itching to tell me I'm wrong.
a) you use more gas in neutral coasting down hill because the engine uses gas to idle, but it does not use more gas because of the gas cutoff when engine braking above 1000-1500 rpm or so. I don't care.
b) Using your engine as a brake seems like a bad idea to me, it's extra wear and tear on the engine, which is much more expensive to replace than a set of brake pads. I'd rather replace the brake pads. And if you downshift to get some engine braking, and you don't double clutch absolutely perfectly, which nobody does, then you're adding even more wear to other expensive and hard to replace parts like the clutch plate and the sychros. Shifting solely to engine brake is a bad idea. No matter how much gas it saves.
c) And here's the kicker nobody ever considers: When you coast down a hill in neutral there's no engine braking, the car will increase its speed (let's disregard wind resistance and tire rolling friction for the moment). This means when you get to the bottom of the hill or start of the next hill you will have more momentum to get farther along it, than you would if you had been saving gas by engine braking. That loss of momentum, means you have to use MORE GAS to get to where you would be if you had just coasted in neutral. But nobody talks about that. And again, I don't care, I just want to know about transmission lubrication....
3) I recently bought a vw golf with an automatic transmission and in the manual it says don't coast down hill in neutral because the transmission is spinning a lot faster because of the wheels on the road than the engine is spinning to lubricate the transmission because it is idling. I wasn't aware that the engine spin is what causing the transmission to be lubricated, but if VW says that's how it is, then that's how it is.
What I was wondering is if the same is true in a manual transmission? (this is a bmw I'm asking about in particular) Can somebody who knows about the design of the lubrication system in a manual transmission explain if it is not going to be lubricated well enough if the engine is at idle or if the engine is disengaged because the clutch pedal is depressed?
While we're at it, they say the tranny fluid in a manual transmission lasts forever. I happen to know that nothing lasts forever, so when are you supposed to change the tranny fluid?