ChuckH is right. Not only do the right-handed muffler bearings require a different type of gas lube, but as we drive on the left, all our roads are cambered off to the left to allow water to drain away when it rains. So we need RHD gas which is weighted with lead to the right to counteract the left-hand camber of our roads. But this did cause a few problems when driving in the other countries of Europe as they drive on the righthand side of the road, same as the US.

Although things have improved a little bit since they took the weight out of the petrol. This new type of petrol is called "unleaded"...