Pulp fiction

THE debate about whether you should use ULP or PULP is interesting, but my experience has left me confused. I own a 1996 Volkswagen Transporter that has been nothing but trouble from day one. On one of the many occasions I took it to a dealership to complain, I was told that if I did not run it on premium unleaded I would invalidate my warranty. Given that there was a sticker on the car that said 91-93 RON, I thought I was within my rights to use ULP without hurting my warranty. Can a VW representative legitimately tell me that if I don't use a higher grade of petrol than is specified on VW's own sticker, then I could lose my warranty?