My BF XR6t recently had the coolant mix with the gearbox oil. Had gearbox rebuild. Got the car running but when shifting down from 5 to 4 there is a very hard gear change, can hear it and feel it through the driveline. Took it back to gearbox shop and they sent the mechatronic from the gearbox to zf to be checked and come back ok. Valve body was ok. They said that I just need to drive it for up to 1000km for the gearbox to relearn and self adapt, does this sound like bullsh*t or is that how it works? It was shifting fine before it went away.