you can use red derv just fine but its not for road use. if you use it on the road you have to declare it and pay the duty. if you don't you will get told off and fined and most probably arrested.
you will also have HMRC on your back for the rest of time and those guys are the least fun people on the planet. they make maggy thatcher look like lee evans!
you can put a different colour dye in the derv but there are certain things in it that allow testers to do a reagent test on it (shows up green) and again if spotted get bum raped.
you can run on veg oil but on modern engines its too thick so will need thinning some how and still risks clogging up verious components, believe me I've done it on an old corsa. then you have to consider that buying new cooking oil isn't worth while, so you have to get second hand which then needs refining and the lumps, fat etc removing from it then thinning down. equipment costs a lot of money to do properly. then again if you use over a certain amount again it has to be declared to HMRC.
its not really worth all the fuss.
you use red derv you have to declare and its no cheaper
use cooking oil you risk engine damage, have to declare and becomes no cheaper and a lot of time and hassle.
was in the garage the other day when someone bought in an old vauxhall 4x4 derv that had been running on veg oil and it had ground to a hault. the bottom of the tank and fuel lines were caked in old chip fat. smelt brilliant although it had been mixed with that smelt like some sort of paint thinners. expensive bill coming their way!
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso