Too much crankcase blow-by is likely caused by:

Worn piston rings are the primary cause of excessive crankcase blow-by. The piston rings create a seal between the piston and the cylinder wall. When these rings become worn or damaged, they cannot maintain an effective seal, allowing combustion gases to escape into the crankcase. This condition, known as blow-by, can lead to increased oil consumption, contamination of the oil, and ultimately, a loss of engine performance.

Understanding the mechanics behind this helps clarify why other factors do not directly lead to increased blow-by. While excessive oil viscosity could affect engine performance and oil circulation, it does not directly impact the integrity of the seal between the piston rings and the cylinder walls. Poor engine timing can lead to inefficient combustion and potential engine knocking but does not specifically cause blow-by. Similarly, incorrect valve clearance mainly affects valve operation and timing rather than the sealing of the piston in the cylinder. Hence, worn piston rings are the primary concern when addressing crankcase blow-by.