GL4 and GL5 are not interchangeable. GL4 is the highest rating for gears. GL5 contains EP additives for hypoid rear end gears. These EP additives contain sulfur which is harmful to copper and brass at any operating temperature. The transmissions originally were designed for straight mineral oil, which means it has no additives. EP or Extreme Pressure lubricant or hypoid lubricant was developed for hypoids. GL4 is a much better lubricant than the old gear boxes were designed for in the first place. As for break in period, I have never heard of a break in period for transmissions.
