* Worn piston rings: Severe wear on the piston rings can cause pieces of the ring material (which sometimes contains a wood-like filler material) to break off and circulate in the oil. This is especially common in engines that have been significantly neglected or run extremely low on oil.
* Wood debris introduced externally: While less likely, it's *possible* some wood debris got into the engine through the air intake (though this would usually be larger pieces and quickly cause much more serious problems). This might occur in unusual circumstances such as an accident or improper repair.
* Contaminated oil: This is less likely to be sawdust itself, but might be similar-looking debris that has gotten into the oil.
Regardless of the exact source, finding sawdust points to serious mechanical issues requiring immediate professional attention. Continued operation of the engine will likely cause catastrophic failure. You should have a mechanic inspect the engine as soon as possible to diagnose the problem and determine the extent of the damage. It might be repairable, but it could also require a rebuild or engine replacement depending on the severity.
