Does your user manual give any guidance? I'd say drive normally for 30 minutes or more (not stop -start) but avoid 6th. On mine that equates to about 40mph / 1000 revs and may not be enough.
Cars are different; personally I'd either RTM, google around to see what people are saying for that particular model or speak to the dealer for proper advice. I can advise you on the Peugeot 2.0 diesel that ends up in a lot of vehicles, but not a Kia.
No, I read the manual from cover to cover last night (yep, i should really get out more) and although it mentioned the light on the dash and what it could be (i.e. the DPF) it didn't give any guidence. I have searched online for my make / model and on the KIA forums people are also baffled.
When we went out last night for a while on the dual carriageway, we stuck to 5th gear. I know on the forums they said you need to get the car to 1200rpm for at least 15 means without roundabouts / stops getting in the way. But where i live its roundabout city, or you get 2 lanes going into 1 etc ... so its bloody hard to do. We did manage it though for about 5 -6 mins before we had to slow down.
I think we have decided to stick to 5th gear on dual carriageways and save 6th for when we are on a motorway. But to be fair, the car is mainly used to ferry me back to work and back (16 miles a day) and an odd trip to the parents / B&Q !!
I am a born worrier, which is why i am asking all these questions.