I had an auxiliary oil-cooler on the 745 too, 'L.S.'. The PO had a caravan and had fitted it for towing. Problem was, it was in addition to, not instead of, the one integral with the rad, so when that failed I still had a problem. It involved changing the ATF (several times), flushing the entire cooling system (several times), replacing all of the hoses, and probably some other things that I have forgotten. The 'box was fine thankfully, and must have gone on to do another 50k or so after that little drama.
Regards, John.