More on GL-5 from a Mercedes site:

http://www.benzworld.org/forums/unimog/1358944-gl4-vs-gl5-gear-oil-plot.html

Here's part of what it says:

"When I first heard this GL-4/GL-5 corrosion thing, I did some research on the oil co web sites. I couldn't find anywhere that said it was harmful, so I sent a tech message to the Amsoil tech guy. His response stated the history of how that myth got started by the ignorant VW people. When GL-5 first came out it was indeed corrosive to brass syncros. But only at temps above 250ºF. Now since it is impossible to get any street driven gear oil to that temp (It is hard to get engine oil that hot!), there will be no problems using regular GL-5. However, just to satisfy the ignorant masses, the API came up with another added rating, called MT-1 which is tacked onto the end of all GL-5 oils you can buy today. With this MT-1, GL-5 oil is not corrosive at any temp."

Until a lubrication engineer or chemical engineer chimes in, we will be reading endless "crackpot" (amateur rather than professional) opinions.

I am a professional engineering geologist and I can only shake my head and roll my eyes at some of what I hear on subjects like climate change.

Ray