bio diesel is ok but straight cooking oil will gum up your piston rings after a while.A lot of people have have different opinions on cooking oil saying that it is ok to use but at the end of the day it is your engine and your decision so if it all goes tits up you can only blame yourself.The 300 tdi disco seems to run very well on bio with no problems at all,but if you use bio make sure you change your fuel filter after a couple of hundred miles as bio is a very good cleaning agent and so clogs your filter.after you change your filter everything should run as normal if not better.