Dave, I'm no expert but I looked into this with a previous car and I don't believe you can run Veg Oil without some modifications, most specifically a heater, regardless of the time of year.
My understanding is the problem is the viscosity of SVO (Straight Vegetable Oil, as it's described by the experts). Older diesel engines with Mechanical Fuel Pumps (i.e, Lucas & Bosch) can run on veg oil without modifications provided the oil is reasonably warm. Most veg oil users still run heaters though, because otherwise the oil thickens and clogs everything up.
I believe the biggest problem with your modern 'commonrail' diesel engine is that you have two fuel pumps - a Lift Pump in the fuel tank, which feeds a High Pressure Pump in the engine bay. I think the problem is the high pressure pump really is that - a jet of fuel from it would probably slice you in two! It would clog up much easier that the older mechanical pumps and would be far more expensive to replace. I'm sure it's possible to run your engine on Veg Oil but I personally wouldn't put any in the tank without any modifications.
I know you wanted advice from someone from experience (which I am not), but I hope that helps.
PS - I would consider Bio-Diesel too. I've heard of decent power gains from people who have run it.