If that doesn't work, think "reboot." IOW, I would disconnect battery, (or..., better..., remove fuse, if there's one for dash instrumentation) wait a few seconds, and reconnect.
Malmsteen, try set->basic settings in the onboard computer to reset all units/settings to the factory default. Usually, the settings are stored in non-volatile memory, so disconnecting the battery or pulling the instrument cluster's fuse is very likely to have no effect.
How do you know that it is stuck? There is no unit displayed in the oil level display. 1 bar of the display equals 0,4l or some weird non-metric unit that is about the same. So, in most cases there should be no change visible in the oil level display if units have been changed. Or do you mean that the level is stuck even though you have filled up oil?
Ah, ok, you're talking about fuel consumption and average speed. It seems that you are just mixing them up. If you want the average speed to be displayed in mph you must set->speedometer to mph in the onboard computer instead of set->consumption to mpg(uk). The latter just affects the fuel consumption display.