[ QUOTE ]
Dave, had you seen that article?
[/ QUOTE ]
I went back & read the article, just glanced over it the first time. I think some folks don't realize that engine oil & gear oil have two different viscosity rating systems. Thus 75w/90 wt gear oil is equivalent in viscosity to 10w/40 wt motor oil etc. I think that engine oil viscosity is rated at 210 degrees & gear gear oil viscosity is rated at 100 degrees.
Redline MT90 & MTL are GL4 rated oils which means that they doe not contain the sulfur compounds found in GL5 rated oils. They have a higher coefficient of friction than a GL5 rated oil. This helps the synchronizer rings in the transmission to work more efficiently.
The sulfur in GL5 rated oil provides necessary protection against the shearing/sliding motion that occurs with hypoid type gears in a rear axle. Hypoid gears are always in a sliding motion. There is very little shearing action & much less friction in the transmission than in the axle so this type of protection is not needed. The sulfur compounds in GL5 tend to damage brass/bronze parts such as the synchro rings in the old Healey transmissions. So the general warning to not use gear oils in Healey transmissions. Many available rear axle oils state that they are acceptable for GL3, GL4, & GL5 usage. These oils obviously contain the sulfur compounds which may damage the brass synchro rings.
Triumph suggested gear oil for a similar transmission, but this was before the advent of available GL5 rated oils which might have caused problems..
D