Originally Posted by JeffBollinger

I called my local GM dealer and they only have GL5 fluid, we need GL4 in our transmissions. GL5 will wear away the brass syncros faster than GL4 because of the phosphor content. GL5 is for hard metals like a rear axle; while GL4 is for softer metals like brass syncros. GL5 bonds so well that when the parts move across each other and shear the oil off the GL5 pulls bits of brass with it. GL4 has about half as much Phosphor and shears more easily leaving the brass alone. The reason for 2 different specs is soft metals and hard metals.