Cooler and transmission oil cooling

Evening. My question is who is familiar with the cooling system. I mean a cooler with additional oil connections. Is that advisable to prevent and use it? Or is that advised off… The gear oil to cool over it. My view is not that the hit or.? Think about it when the water leaks cooling run and gets into the part of the oil circuit. And you don’t notice that, it does a lot of damage to the gear. Or? Isn’t it better a n External oil cooler to use. ? Or do I see this wrong. Mh ??