LC 100 sludge.

Hi All
This is my first submission to the forum after many years or looking, learning, laughing and lamenting. Thanks to all those contributors.
Now to the serious stuff. I have a 2002 model 100 series factory standard 1HD-FTE Turbo'd Intercooled GXL with snorkle.
It has travelled 300,000 Klms mostly highway stuff (60%). Some stop start city stuff(30%). Some heavy going (10%). The oil I use is Castrol diesel 15w40. Changed at 10,000k intervals. The vehicle has nearly always been run to working temperature before shutting down. It uses no oil. It starts very easily. It blows no unburned fuel smoke. It idles very smoothly. It pulls like it should. It drinks 10% more fuel now than it did at 100,000ks.
I decided to shout it an injector service but upon removal of the inlet manifold I discovered a fair lining of sludge in the manifold(2-3mm thick). It is clean under the top end cover and around the cam and valve springs i.e. no sludge.
My question to the group is; How and why did this sludge build up happen and has any one had a similar experience?
Thanks in advance
Dave