Oil

Oil is the lubricating fluid that your engine needs in order to function correctly. On an aircooled engine it performs 3 functions:
*Lubrication - reduces wear on metal parts
*Cooling - helps to carry heat away from the heads
*Cleaning - keeps harmfull deposites in suspension until they can be filtered out (on those engines with oil filters)
For more oil topics see:
*Changing the oil
*Discussion of oil