It is pretty relaxed as it's basically checking that smoke will not restrict visibility for yourself or other drivers. Do not forget that there are faulty cars and some remapped cars that has been mapped in a way that allows for more smoke. And also soot accumulates in exhaust and then is blown out when you boot it.

I had my DPF removed after 5 years, 180,000 miles due it being permanently blocked.

A local garage removed it and smashed the internals out which consisted of a pre-cat and DPF and a pile of soot.

They then refitted it and had it remapped with DPF delete by a mobile agent.

The DPF measuring blocks in VCDS are just blank now.

There was never any smoke when the DPF was working and the tailpipes were still clean inside after 180,000 miles but it is the same as a Euro 3 car now, smoke seen in the mirror when accelerating swiftly and soot in the exhaust tips.

at least it didn't stop under a wooden bridge this time!!! (D9000 )
I have used Fully Synthetic 0w30-5W40 Mobil 1 in a Senator 24v for 18 years and 189000miles with no problems at all. As said previously change the oil on a regular basis 18000 miles seems a long time to me, I'll be changing it at 9-11000miles.
My 4.2TDI needs VW507.00 spec with the DPF. Audi charged £150 for oil in my last service so I will be supplying my own next time.
Check out Euro car parts, discount code varies 31% off just finished now on 25% off. Opie oils is another keenly priced site, or if you have a vauxhall trade club number GM brand 507.00 is available.

TPS were doing a deal on 20 litres of low ash oil a couple of weeks ago for around £70 plus VAT