I've been doing a little bit of homework here.
Driving on the highway for 30 minutes every week, or even every other day is not going to be enough to keep the DPF clear just by itself.
The reason for this is that soot burns off very slowly at the temperatures normally cruising on the highway. (8th gear and 110kph is not really loading the engine very much at all, is it?). The 8 speed box is great for economy but since we're not burning so much fuel the EGT (exhaust gas temp) is low.
To get the temps up high enough you have to be driving with the engine under load like going up a very long hill and or towing something heavy up hill. Even then it would not be sustained long enough to fully clear the filter.
This diagram i found shows how fast "an amount" of soot oxidises at various temperatures. This test was probably done in lab and not in a truck but the chemistry is the same.
600 C - very fast and complete soot burn, 400 C - pretty slow and only partial burn
I drove about 470km on a round trip on the highway last Sunday.
I had the OBDLink reader running and monitoring the CAT temp1. (PID 3C)
The app i am using doesnt decode the EGT properly. (PID 78 is 9bytes of data, it might be looking at the wrong part).
There seems to be 3 ? temp sensors in the exhaust but I haven't worked them out yet.
I found another android app which reads EGT1 and seemingly some DPF data (yet to be verified) but it keeps crashing on me after a minute or so.
The EGT sensor varies very rapidly with engine load. I've seen temps 200 to over 500C within several seconds.
The CAT temp is a more stable value which is sort of representative of the whole system.
Idling in traffic stopped = 200 C
Driving in normal urban traffic flow gently up to 60kph = 200 - 250C
Driving in normal traffic flowing up to 80kph = can go up to 300 C
Highway drivng 110kph in 8th on flat = 300-350 C
Highway driving up and incline (kicks down to 7th) = up to 450 C
Highway driving downhill foot off throttle = cools down to below 150 C.
Driving in 6th or 7th at higher revs doesn't increase the temp either as it is the load and how much fuel is injected that raises the temps.
Higher revs with light load pumps more airflow and actually cools it down.
In 4hrs of highway driving i only saw it go over 450C for a cumulative time of a few minutes. Most of the time it was only 270 - 350 C range.
Note: I was not being a leadfoot, just normal driving (it was double demerits on the weekend remember).
Flooring the pedal does bring the temps up more of course but impossible to keep them high and also keep your drivers license.
To burn the the soot of really quickly, 600 C is where it needs to be.
Maybe the DPF temp is a bit higher than i'm measuring at the CAT but keeping it high enough long enough to burn completely clear i'm not so sure about that.
If it cant clean just driving and and get up to 600 C it'll eventually have to do a regen, injecting fuel in the exhaust. This is fine if it works as it should, burns the soot off quickly and effectivley and it doesn't go limp mode. If the software works properly it and triggers it correctly it should do it.
I think even driving on the highway some of the time you would still be accumulating soot in the DPF rather than clearing it out.
I've found a parameter on a docunment the SAE.org site (thanks google) that gives some sort of calculated value % full status, but waiting to see if it is reading properly with meaningful values. Every time I check the parameter value it is a little bit higher, filter is slowly filling up
Basically i'm waiting for it to regen (again?), and see when and what it does.