I believe a 10-40w or 15-50w is recommended, and if you search there are arguaments as to synthetic/semi-synthetic oils. I use semi-saynethetic in mine as its older, and i'll probably go for whichever's cheapest in Halfords at the time! If anyone can recommend which range of viscosity is best or some good brands please post as mine's due soon too!
As for DIY'ing it. I've got a haynes and it looks very straight forward, basically:
Remove the filter (front left of the engine bay)
Remove the drain plug (left had side of the sump)
Leave to drain (probably best to catch it in something!)
Re-fit sump plug with new seal
Fill up with oil (5.8L)
Fit new filter
Run for a few minutes
Check level and add more oil if necessary
Drive it like you stole it
This is for the 4-cylinder petrol engines, and so your V6 might have the bits in slightly different places, might take more oil too thinking about it, but those are the general steps.
Good luck, deff. a project for the driveway mechanic
Anyone with a c240 confirm its a similar process?