I have a 90 with a defender 200tdi engine.
I have been running a varying mix for the last 4 months.
I started on a 50/50 mix of pump diesel and rapeseed oil from tesco. This was ok in the colder months although the only thing I noticed was the fact that on starting it was a little lumpier untill it was warm. Once my engine is warm I notice no difference whatsoever.
Tesco was the cheapest for me to purchase rapeseed oil (sold as veg oil). I can get it in my tesco extra store for 10Litres (2x5litres bottles) for £9, although 2 weeks ago they dropped it to £8.
I have only tried one other outlet, Makro.
I paid £18 for 20 litres in Makro, which is slightly more expensive but I can buy it in sheer quantity from there, unlike tesco.
Recently my ratio mix has change from about 50/50 to almost 90/10.
That being 90 parts rapeseed oil to 10 parts diesel.
All roughly measured by filling the tank with appropriate amounts.
Tomorrow I shall be putting in straight veg oil, roughly 40 litres, and a slight drop of petrol to see what that mix runs like. I shall get back to you with my results.
So far though, 4 months of running veg oil with no problems.
All the best,
Matt.