The only thing I see with veg oil or Wd-40, is that it attracts the dirt, whereas the slate oil claims to repel it. I definitely wouldn't put veg oil on it. Look how it goes on the paint below the filler cap.
I've never used the slate oil myself, but have seen slate done with it once or twice. Hotspot are an old company, their stove blacking paste is great stuff. i use quite a lot of it. I'd trust them.
Hmm...
Just called Hotspot, and said that i was just about to start with WD40, and someone suggested i called them, and i wanted them to convince me..
They put me on hold for a moment, and came back and said "our product has been tested on slate, and obviously we haven't tested wd40, so we cant say any more"
...which wasnt entirely what i was expecting. I wanted them to say "Our product is good because..."
Ive just looked up the COSH sheet -
Its naptha (lighter fluid, IIRC), white spirit, and "Metallic Driers"
The naptha and white spirit are probably mainly carriers, but i wonder what the "Metalic Dryers" are?