if its a 1.8 petty then as above id suspect a head going.
if its a TD4 then its pretty rare the heads going - if you are getting sludge in the tank and its brown not black id suspect a possible hole in your transmission or gearbox cooler.
May be worth removing the filler plug on your IRD or gearbox to see if a watery sludge pours out...usually you would only get a few drips of oil out as the plug level is the actual fill level.
My mates TD4 did the same but the sludge was black and he wasnt losing water or had any unusaly engine/smoke behaviour which pointed to his engine oil cooler had a slight hole in it.
Hope this helps,