I know of a couple guys filtering waste oil and mixing it with thier fuel.
my cousin works with a guy mixing 50/50 in his 97 and says his milage is same or better. also know a friend with a 80 something rabbit diesel hes been blending waste oil for years. ive got a jetta tdi also and thinking about trying it. just looking for any input from others who have done this.

 

This has been going on for decades in all sorts of diesels , Hwy & equipment .
The problems start with what oil is being used [ not ATF ect. ] , then getting clean enough [ removing particals , water and acids ] .
Then one of the last problems I see is that they are manufacturing the newer systems to be fuel specific [ less tolerant of alternate fuels , requiring only ULSD ] .
That is just one reason that I would not buy a newer truck , but I would like [ if I could afford , a P-pumped 24 v ] , always trying to have my cake & eat it too .

 

it depends on the year tdi you have. i know the newer ones can get clogged up on B20 alone. the older tdis shouldnt have too much of a problem running a waste oil blend. i usually put in 1-2 gallons per fill up on my truck, the only thing i notice is egt's are a little higher (avg 50*)

 

my tdi jetta is a 2000 model has all mechanical pump and injectors.
as for my truck the common rail i dont think would tolerate that at all.

 

you should be ok on the tdi then. i would try it in small amounts just to see how it affects the car