Can anyone shed a little light on this subject for me? Whats the difference between a gl-4 and gl-5 synthetic gear oil? Which, if any, are bad for my car and why? I already have 3 qts. of mobil 1 which is a gl-5 gear oil...will this really damage my syncros or is it ok? or should I go with the Redline MT-90 which is a gl-4?