If you have a loader, lift up the side opposite the fill to level plug... remove that plug and allow it to drain. If you don't have a loader or you do not want to lift it use a suction pump, either electric or manual, either method works well, I've used both just remember to have something attached to the 3pt for ballast if you use the loader to lift it.
I switched my gear boxes to synthetic lube, they all are running a lot cooler. The heavier wt. lubes like you have are very good for summer mower gears. May be too thick for winter, snow blower use... KennyV