David
We have used numerous types of T seal over the evolution of this engine design through from S2 all the way to 300TDI. There has been no repeating theme of any particular type of material T seal being troublesome.
With the Pioneer Weston seal kit we specifically asked for this type of T seal to be included I.e. neoprene type T seal which may look like cork.
Land Rover used silicone rubber for a while with the 2.5 diesel instead of T seals and abandoned this in favour of a neoprene type T seals. It was not a guarantee of success using silicone rubber only.
We have found over many years of supplying this type of rear oil seal and others that when a customer experiences repetitive failure it is generally isolated to that customer and not a repeating theme which would suggest that other factors may be in play.
The half plates on this engine type must be in good condition and if memory serves they cover a large percentage of the joint between the main bearing cap and the block casting. The split type seal is a type that was abandoned by Land Rover and changed to a lip seal for 5mb engines for obvious reasons.
As you rightfully state the rear oil seal on this engine type can be a challenge to all levels of ability.
Richard Turner