I’m currently using Redline MTL/MT-90 mix for gearbox oil. I am due for a change and not too impressed with the redline for fixing the notchiness, it improved but its still there. Been doing some research and I have read good things about GM/Pennzoil synchromesh. But what I have not read is anyone taking into account the oil weight. Mazda recommends GL-4 75w90 for my Mx6. Synchromesh is a modified 5w30 oil and I'm just wondering if it offers enough protection against wear at operating temperature. Take a look at the specs: Synchromesh (75w80?) / Pennzoil GL-4 75w90 Synthetic / MTL (70w80) / MT90 75w90 Flash Point oF: 385 445 450 465 Pour Point oF: -50 -44 -58 -49 Viscosity @ 40o C cSt: 41.6 108 56.2 90.0 @ 100o C cSt: 9.08 15.3 10.6 15.6 @ 100o F SUS: 209.4 554 @ 210o F SUS: 56.7 80.7 I've read that it improves shifting smoothness even over the Redline fluids, although will it harm anything if I use it? What temp do gearboxes run at anyways?