I can now recommend the Amsoil 20W-50 motorcycle oil (MCV) for a manual gearbox under certain conditions. I have used this oil for several months in a badly worn, high mileage Mitsubishi 5 speed in an old Ford Ranger that I do not want to tear into for a rebuild. The heavier oil has quieted the noisy bearings/gears and has not interfered with shifting. My reason for using this oil, even though not recommended and sort of expensive, was that it is a GL-4 rated gear oil, yet uses no friction modifiers to cause problems with the synchros. And it is a very shear-stable oil that will hold it's viscosity well. I don't expect to have any cold weather shifting woes, especially since it rarely goes below freezing here in the winter. We'll see when the time comes. So far, so good. Might be overkill, but it's working as I had hoped in this situation. The factory recommended fluid is basically a 10W-40 motor oil, so this is just one grade up the chart for a unit with nearly 300k miles.
