I change the oil & filter on mine every 7.5k at the latest (Try to do every 5k). I use a good quality lube (the same oil as many Nissan Skyline owners use - it is an ACEA-B3 oil, so is compatible - If you need to know its GM/Vauxhall 10w-40 Semi-Synthetic or if you can't get this cheap enough (I have a trade card, then use Comma oil, its made by the same people) and a new filter every time. Costs me just under £20 to change both.
Don't listen to the 12k service light, I think that is too far for a petrol, let alone a diesel.