That looks like a Hurth ZF150 MIV..? I have the same gear. Mine has always had the cooler and the engine meter has 3400 hours.. I change the gear fluid twice per season and do a flush at the end of each season.. I also over fill it for winter right up to the fill rim. Those are a PITA to re-build and require special tools made only for that gear... Who did you use? Hansen?