And there was me thinking that a double negative makes a positive, hence it might make a 2.5 more reliable!!
Seriously it aint worth risking mate....
A few years ago when you could buy 25 litres of new cooking oil for £12 it was worth a gamble on an old car as if you could keep it going for 6months you were quids in and the car had paid for itself several times over, but now its really not worth it. Older 4x4's are getting scarcer due to good scrap prices and hence more money for the ones left, and everyones in on it with the biofuel, so chippys that used to pay you to take old oil now want money for it...
Best advise is just drive it nicely and make sure its well maintained to give max mpg...try and stick to 65-70 tops on motorways and try to avoid stopping your 2 tonne car by timing it right with traffic lights...You should get a fuel return then that is comparable with some really fancy later 4x4's with all their electronic ecorubbish, only difference is that yours isnt depreciating by £5k a year!!!