1.8T only takes 3.5L of oil, so you'll only want to drain out a litre or so and refil with a similar amount of diesel.
Personally, i'd keep the revs above idle. If the oil pump is a bit dodgy, then idle with thin oil could easily have the pressure dropping below 1 bar, my old engine used to drop as low as 8psi on hot idle, and thats with proper oil in it.
I'd say 1500rpm baseline, and upto 2000rpm every few mins for 30seconds, i wouldnt take it too much above 2000, as the turbo will begin to spool around there and you dont really want that on the flushing mix.
Then do a normal oil change, and refil with 5w40 synth oil and a new filter. Keep an eye on the new oil, it might go black very quickly, and if it does you'll want to do another change after 500 or 1000miels or so. If it stays golden then change it around every 5k like normal.