If you don't know specifically what went into the tank, it's pretty pointless to speculate.
As far as diesel 240s go, switching from old diesel to ULSD is probably the single worst thing you can do for fuel seals, and that happened years ago.
Biodiesel swells rubber seals which can keep them from leaking, veg oil does not. Veg, bio, and old diesel all lube the pump, ULSD doesn't really, so a lubricity additive is a good idea for old style pumps.
Your best bet is run a clear return line so you can see if there are bubble, and if there is a leak, you fix it, no biggie. I suggest getting 4 feet of the small return hose form a VW shop and replace all the little hoops between the injectors, its cheap and they tend to leak if even looked at funny ~2 years on bio, maybe 5 on dino.