The SX4 MY09 owner manual strongly recommends the GL-4 75W90 gear oil. My dealer put the GL-5 (75W90) as the GL-4 was declared “obsolete” for North America!? Is it safe for the synchros/bushings? I do not believe so but need any expert’s opinion. There is a field in some technical data sheets labeled “Copper Corrosion - 1B”: is this the only specific characteristic/data to look for/compare?