fermin, you referred to the washers that BMW keeps saying to replace all the time.
Those are the crush washers (or "crush" washers, depending on what kind you get) that are used to seal the oil drain and fill caps.
The ones I like the best are copper and hollow in the middle, and actually
crush.
The other kind are aluminum, and are of square section, and they might be o.k., but I really like the copper!
You can get them here:
http://www.bmwhucky.com/007533.html