After replacing my radiator on Saturday I thought I'd break the old one apart to see if failure resulting in gearbox contamination was close, also I was interested to see the inside.
It seems that the actual cooler was fine, so it's my guess that the problems start with the connections. When I unscrewed the unions they didn't look that great, one came apart far too easily for my liking. There's also an O ring inside. There was a fair amount of suspect looking gunge and some rust around these parts.
So, if like me you want the peace of mind that you won't suffer this problem, don't do what I did and fit a separate cooler, just fit a new radiator, it's cheap and it's easy.
As you can see, my radiator was in poor condition so it must have lost some of its cooling ability
.