Many manual gearboxes depend on synchronizer friction to match shaft speeds. High‑sulfur/phosphorus EP packages typical of GL‑5 gear oils can lower that friction and may stain yellow metals, so shifting can feel slower even when viscosity is correct. That’s why many manuals specify GL‑4. Some modern “GL‑4/5” blends balance EP protection with synchronizer friendliness for mixed fleets or transaxles, but the owner’s manual still leads. In Nordic use, cold mornings amplify this: reduced friction at low temperature is felt at the lever. The same principle applies to 75w90 GL-4/5.