If you go for Magnatec you can get 8 litres in Costco for £35.
They also sell Chevron Supreme 10w40 for £44 for 20 litres.
I personally don't run semi synth oil longer than 5/6k miles very often, call me paranoid!http://www.inlinefilters.co.uk
I think that is the correct address do Baldwin oil filters B178 for £2:83 and fuel filters for unde a fiver, much better price than Bosch at EuroCarParts which are (before online discount) £8 and £22. Mobil 1 New Life or Turbo Diesel 0w40 are both £30 for 5 litres, Castrol Edge the same for 4 litres.
If you run a CI 4 oil you will be protecting your engine a little bit better.
Shell Rimula R6 LM 10w40 or Mobil Delvac XHP LE 10w40 are available from distributors for around £80/90 for 20 litres but are meant for long oil change intervals so should laugh at 10k in a VM, wether it be 2.5 or 2.8.
My 2.5 VM is running sweat on Mobil 1 New Life 0w40, this is the newer SN version, previously it was just SM, what difference there is I don't know.
I have run Chevron Supreme for 7k before, just changed Valvolene Maxlife Diesel 10w40 synth blend which was in 8k.
Any good oil will see you right, I would not run conventional/dino/mineral oil in a VM lump.
If you do a lot of local running about consider doing shorter oil change intervals.
Approved viscosities are 0w40, 5w40 and 10w40. That is straight from an Email sent to VM themselves. B3, B4 or CI 4. They actually say you can use B5 aswell but i have never found a B5 oil that is -w40.