Oil Cooling for Automatics when towing

Last year the auto box on my 10 year old BMW X5 failed. At the time I put it down to chance but now I realised that towing a much heavier van across Europe may have been a contributory factor. I recently met a mechanical engineer who had worked in the automotive industry and he insisted that additional oil cooling is essential if towing.

My problem is that I am considering buying a new towcar (possibly a 3 litre X3) and I am concerned that the normal oil cooling would be insufficient.  Any thoughts?