Besides lubrication, there are two other properties of the lubricant that can affect long term preformance and life of a transmission. One is compatibility with the seals. The wrong fluid may harden, soften, or swell the seals.

The other is its effect on brass components. I work with lubrication engineers and one of the lubricant properties tested in our lab is the copper strip corrosion test. Lubricant cannot be too aggressive on brass if brass is present in the unit. Last friday I had a good discussion about scuffing of materials in the presence of a lubricant. If the lubricant doesn't have the proper antiscuffing agents or is chemically reactive with brass, over time it can destroy the transmission. The synchronizer mechanism has unique demands of the lubricant because it IS brass and it depends on frictional contact with the mating cone surface.

So you can see a lot of factors are considered when specifying a lubricant for a particular transmission. A lot of people get into trouble when they try to reengineer a car without knowing what they are doing. Repair your transmission and stick with the specified lubricant.

If I read between the lines correctly, you ran gear oil for a long time before switching to ATF. I suspect the synchronizers were being slowly damaged during the time you used gear oil, and the problem now cropped up when - only by coincidence - you switched to ATF. Or the problem is unrelated to lubrication. If the problem goes away by switching back to gear oil, more power to you. But I suspect the problem will stay.

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95 E320 Cabriolet, 169K