It wouldn't surprise me if the dealer cleaned the dpf prior to the sale as they don't do many miles before they clog up again.
Best bet is to have it removed and egr blanked and mapped out.
What mileage does it have?
Mines on 201167 now, did an oil and filter change yesterday aiming for every 6k using 0w30 A5/B5.
I've also replaced 2 rear calipers and brake fluid last year and both upper front control arms replaced.
Still does 50 mpg and sounds smooth. A puff of white smoke is apparent on startup in this...
My cars aircon never worked since buying it 5 years ago.
I just live with it, haven't tried a re-gas so can't help.
how often is it needed in our climate.
I wouldn't use thicker oil, especially in this climate. The engine needs all the help it gets from immediate lubrication.
I use a 5W30 in mine and seems to work
Take a look here:
https://justgermanparts.co.uk/a-guide-to-cylinder-head-gasket-thickness/?srsltid=AfmBOorBFk0iLlQ5Y2XKrTuV7StoEiGiZs2yy0VmpPFYZzn2Wu4n8k46
a lot of cars suffer in this country due to the constant damp and muddy roads and the salt spread during freezing temperatures.
There are some good products to combat this but require intensive work.
Used this on a sprinter I have (renowned for rusting) and it was great...