I noticed similiar problems after filling up at BP Carine a while back. Just didnt seem "right" and the even the bloke behind the counter said he doesnt sell a lot of 98....

How long does fuel take to go off? If its been sitting in the tanks for a while, surely its a possibility?

EDIT. Thinking back to it, using a generator 250kms past Esperance last week we ran into some problems with off fuel.. The generator works fine normally but my mate started using fuel that had been sitting in a jerry can in his garage for the last 6-8mths... Ran alright for a little while but eventually it spat the dummy!