The coolers are pretty reliable and don't seem to choke up unless, as in my case, the cooling system was full of rubbish from bodged attempts to seal a leaky matrix. The filter stops them getting choked.
Good job they're reliable. They cost an eyewatering sum of money new. In excess of £200
That oil looked well used Lexi..
It's easy to see how the filter becomes choked over time as there'll be a lot of dust and debris suspended in that sample, a good deal of which plugs the filter over time.
That oil was better out than in, that's for sure!
The boxes do run hot and this does not help anything. Actually, all the 4HP20s I've stripped to date don't show a huge amount of clutch plate wear. They either seem to be OK or totally wrecked due to something else failing.
I reckon that with regular oil changes and a filter replacement every now and again, the 'box will live forever. They really are a work of solid engineering; it's just such a shame the filter is not easy to replace...