On a 2.0 tdci  163 bhp focus when the car is in regen mode where is the extra fuel being pumped into?  If its going into the combustion chambers yes I can see that some will find its way into the engine.  On the other hand if its going into the exhaust it will not be a problem. I did a regen using forscan last week and the difference is night and day. No increase in oil level much better performance and the on board mpg indicator  has gone from 45 to 55.  I will check the actual mpg next time I fill up.

You would find that countless failed/aborted regens from repeated short journeys may cause an increase in oil level, but a single regen event won't make a lot of difference to oil level. The fuel gets dumped into the exhaust via the cylinders, through the DPF, the idea being that the extra fuel will make the DPF hotter to burn off all the crap. 

Similar in principle with a petrol engines when cats get damaged from misfires, the excessive raw fuel going through the cat causes it to get too hot and in their case causes damage. 

But with DPFs the extra fuel helps burn off the crap

Thanks. Makes sense.  Does the extra fuel ignite in the dpf/exhaust or in the combustion chambers. The exhaust gasses are certainly very hot during the process.

Depends which engine you have.  With 'post injection' regen, the fuel starts in the cylinders.  On a car with a DPF vapouriser, the diesel is injected directly into the DPF, bypassing the cylinders altogether.  

Having said that, cars with a vapouriser may also use some post injection to get exhaust temps up before the vapouriser injection, as I have seen reports of oil level increase on vapouriser equipped cars.