I apologise if this is old news to people, but I did a fair bit of investigation on DPFs after I bought my Mondeo, and it appears that there are three types;
cDPF (c=coated). This is the type fitted to the MK4 Mondeo TDCi (Peugot engine, apparently). This type "regenerates" as / when required by heating up to burn out the ash that has accumulated in the filter. This process does not use any additional additives.
Additive type. Used on Ford Focus diesels (around 2008) and presumably many other cars. A sensor in the exhaust system measures the internal exhaust pressure, and at the specified limit injects a small dose of additive into the DPF where it ignites, elevating the temperature of the DPF to remove the trapped ash. This additive apparently requires replenishment every 37.5K and is horribly expensive.
Fuel type. I believe this is the type used by Mazda, Mercedes and others. Essentially, a small amount of diesel fuel is deliberately injected into the cylinder on the exhaust stroke. This fuel is not burnt in the cylinder, but is forced out past the exhaust valves and enters the exhaust system. When the exhaust gets up to temperature, this fuel is combusted, raising the exhaust / DPF temperature to burn out the ash. This is presumably why you would need to maintain engine RPM at 2000+. I don't know whether the fuel is injected on every exhaust stroke.
The problem with this method is that some of the fuel is forced past the piston rings and into the sump due to the internal cylinder pressure. A few years ago, this would have been fine, as the diesel fuel was sufficiently volatile to simply evaporate away. However, the government (obviously experts in such matters) decided to increase the permissible level of "bio-fuel" in diesel. Unfortunately, bio-fuel isn't particularly volatile, and doesn't evaporate away - it just sits in the sump contaminating your engine oil and, more importantly, continually adding to the volume of fluid in the engine. As the volume increases, so does the crank-case pressure. The obvious down-side of running at higher revs to raise the exhaust temperature is that more fuel will be forced past the rings into the sump.
As the pressure increases, if the fluid level is high enough, oil / bio-fuel mix can be forced upward back past the piston rings where it ignites. At least, that's how I understand it - I'm quite happy to be proved wrong (I want a Mazda 6 for my next car!).