Not sure why your talking "GL5" oils?Changed my recently changed transmission fluid (<1500k) today to redline. From the redline site.
"Since most GL-5 gear oils for differentials are too slippery for manual transmissions, Red Line offers these products that ensure proper shifting while providing ultimate wear protection"
Note the too slippery thingy quote.
Not to impressed with the oil price though, $90+.
I did notice trans slippage in cooler morning temps, 0c until the trans warmed up then the slippage was gone.
Should only be a "GL4" rated oil!