Re: Oil Drain Plug Washer
Most auto parts stores will carry a range of nylon and aluminum sealing washers. I prefer the nylon ones for my British cars as they take less torque to seal. Just get one that fits the plug and you should be good to go. Don't forget to calculate the closest metric size as most the generic washers will be metric. Take a look at what's on the shelf. Some of the metric nylon ones will actually fit sizes they shouldn't (i.e. a 14mm nylon washer will fit 9/16" drain plugs).
The Mini, however, has a rather large plug going into an alumium gearbox/sump. I use a MotorMite bonded rubber/metal seal on it so I don't have to torque the thing much to get a seal.