ive read on a couple of posts on here about using old chip oil / engine oil as an additive to normal diesel.

my main questions are:

can these "additives" damage the engine in ANY way (my disco is my only transport and cant afford to knacker it...)

and if theyre safe... what ratio do you add them?

my disco is a 1996 300tdi auto

In this post, the subject was done to death:
http://www.landyzone.co.uk/lz/f7/cooking-oil-td5-80487-6.html

We all draw our own conclusions. Personally, I often use clean old oil (from oil changes in the family's cars and motorbikes). I dilute heavily (around 1 litre per tank).

IMHO my TD5 runs better! Also saving £1.12 on every tank, and saving a trip to the council tip to take old oil for recycling - what's not to like there!

And just to be sure, I carry a spare fuel filter but not needed it yet!

I run almost anything through mine .. chip oil, gearbox oil, engine oil, 2 stroke oil .. all filtered through a fuel filter and a nylon filled tube (like a fish-tank filter) .. and never to more than 50% ish. Mostly I run it on 'normal' diesel and veg oil straight off the shelf!

No issues at all other than smoother, quieter running ..