Quote:
Originally Posted by Rory McNeil
I`m curious as well. I had been told that the main difference between GL-4 and GL-5 gear oils, was that the GL-4 was designed for use in manual transmissions with "yellow metal" syncronizers, like brass/bronze, and the GL-5 was for syncros with paper or carbon fiber linings. It seems that finding GL-4 gear oil is not as easy these days, GL-5 seems far more common. For the Toploader 4 speed in my street car, I have been using GL-4 Brad Penn, seems most of the major oil companies have just GL-5.
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Rory,
GL-5 has a higher EP (Extreme Pressure) rating than GL-4 and it offers a higher shock load protection when you have a high angle of gear contact. GL-4 gear oil, no matter the viscosity, has half of the sulfur/phosphorous additive that gives the shock load protection.
Nevertheless, because of the different angle of gear contact and softer contact material, on a manual transmission, this same additive used on the GL-5 gear oil effects the syncros and other soft metal components by fretting the contact surface.