Who drives around in smelly dirty and slow oil burning diesels these days?
Err....that would be me :-)
We've had this discussion about waste oil before. I tend to use new (£1 litre), can use it straight in my old Pajero (pig to start on cold mornings) or mixed in my Voyager (Mk1 will run on straight, Mk2 & 3 need to be mixed, a lot).
Newer engines won't run on it (some won't run on Bio either) you just need to check, hundreds of sites out there will tell you which/how etc.
If you can get it from your local chip shop or Indian takeaway great. Time was when they'd PAY YOU to take it, now most know it's for fuel and want back almost what they paid for it new.
It 'can' screw up your engine, depending how you use it and what you use it in. I found that adding white spirit to the oil thinned it down for easier starting in the Pajero (you do have to pay tax on white spirit used....no really, you do, I have the receipts).
Go into it with your eyes open, it can save money, don't kid yourself you're saving the planet and it's all green, it's about the money, if you wanted to save the planet you'd have bought a bike :-)