Oily Riesling is a fault
Michel Chapoutier, the renowned Rhone producer, said recently that Riesling should never smell of petrol and that it is a result of a mistake during the winemaking process. The petrol characteristic, which is often prized amongst Riesling aficionados, is a result of decomposition of the veins within the grape he says, and these veins become more fragile as the grape matures. The vital aspect of Riesling vinification is the gentlest of pressings, often for 12 hours, to avoid the breakdown of the vascular structure within the grape.
Chapoutier likens the debate over petrol aromas in Riesling to the issue of brettanomyces, saying it was absurd that historical defects in wine should be accepted as part of the character of the wine.