@Levin
Generally, manufacturers give guidelines for using bio-diesel blends in their diesel engines. I inquired about is of a sales rep for Volvo (trucks division), and he said upto 30% veggie oil is OK with that particular diesel engine (might be different for other engines). Higher than that, and you'll have problems of gumming, bacterial growth, failure of anti-pollution devices and so forth. In short, the vehicle wasn't designed to run higher proportions of veggie oil. The truck in question was Euro-5 compliant and had common-rail FI similar to the D4-D.
As for the D4-D engines, I wouldn't recommend it. A small percentage might be OK, but in the event of failure, you won't be covered under warranty FWIW.
Older engines cope better with vegetable oil as their fueling system is less precise and tougher.
@kutza
As far as I know, you need to change the hoses when you run ethanol or methanol / petrol blends. As it's the alcohol which attacks rubber hoses.
Cheers!
Sami