I had been using some Lucas fuel additive in my 88 Jetta Diesel.
I think it's really just some sort of Veggie oil....for $7/Quart
Last tank, (12 gallons) I put in a quart of 10-30 motor oil.
The engine is smoother running, runs cooler, and has more power.
I thought VW suggested 10 (TEN) percent gas in winter NOT 30(!!) percent.
I asked at the Diesel injection shop years ago, and they said "VW suggests 10% but
BOSCH suggests 0%"......
dk