I have gotten very good results discussing this with Lube manufacturers. Most
have tech lines that are availble free (some are even 800 numbers, look on the
web site for your favorite manufacturer). I spoke with a rep from Amsoil who
had some very specific info. He claimed that the Additive package in GL 4 and
GL-5 is the same there is just twice as much in the GL 5. The additive package
in GL-5 is only harmful to yellow metals and only at temperatures above 215
degrees. He did not explain why the same package in GL4 is not harmful (I
assume it is a reaction that does not occur at lower concentrations or it is
slow enough it is insignificant.
The Mobil rep told me not to use the Mobil 1 gear lube I had just purchased.
And that they did not make a GL 4 gear lube.
BTW Redline seems to strongly recommend their GL 4 package (MT-90 and MT) for
most Manual Transmission applications (not just older cars).
Another alternative is straight weight motor oil 40-50w. Multi Viscosity is
not recommended for Transmissions as the gears tend to cut the Viscosity Index
improvers, But why bother when Redline makes an oil specifically for this
application.
Tom Leake
65 TR4 A IRS
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