Large oil sumps (7-8L) mean twice as long oil drain intervals compared
to small (3.7-4.0L). It was all done for the fleet buyer who basically
wanted a) minimal servicing costs and b) resale on the used market to
be close to what they bought at on the subsidised fleet bulk buyer
market.

Nothing to stop someone using a fleet oil from a motor factor like
Comma, it will be a fair bit less. Note generic oil filters often have
different bypass oil pressure compared to OEM. This can raise its head
during cold starts etc.

Check your air filter if getting a bit forgetful, the rubber seal DOES
break down with age and go through the engine & lodge in the cat. I
know this because of the smell of burning rubber once and it was not
my tyres... had completely forgotten... must have been 8yrs rather
than the suggested 2yrs. Mileage wise it was right, but age wise
things do deteriorate. Likewise coolant level is easy to "drift off to
the back burner" when something somewhere gets a tiny bit leaky (crack
in a plastic fitting which only propogates over years and only opens
at certain temperatures).

Modern diesels are very hard on their oil, the particulate buildup is
severe and multiple short trips or poor monitoring of oil level can
result in very big bills. Things are also not well made, intercoolers
are the thinnest aluminium they could get to come out of the factory
without imploding on impact with a neutrino.