Why does engine oil consumption increase as oil ages?

[movin]

My '87 seems to consume very little oil after a change (doesn't smoke at all), then after about 500 to 700 miles it begins to increase the rate of consumption (& smokes a little at start-up). With 122,000mi I know the guides and cylinders may be worn, (those stupid Alusil cylinders!). One theory I have is that the cylinders are worn and blow-by contaminates are getting into the oil supply. I noticed during oil changes that the oil tank drain plug was covered with a coating of black soot, that wasn't noticeable 20,000+mi ago. Any thoughts?

[ruf-porsche]

Gasoline from blowby dilute the oil so the viscosity is less and with the reduce viscosity it can enter the combustion chamber of the engine thru the valve guides and rings.

The blowby is actually routed thru the oil tank from the oil breather on top of the engine.

[bsaffin]

Movin,
What is your consumption between oil changes? And what oil/viscositity do you use?

...Bruce

[movin]

I was using Red Line or Mobil 1 15W-50, but it was using a quart in less than 500mi. Someone suggested switching back to a non-synthectic, so I used Kendall 20W-50. Right away I noticed the smoking at startup decreased, but that's been my experience with Mobil 1 or Red Line, change the oil and the consumption goes way down for a time. Then, after a while the consumption starts to increase until I change the oil again. At least with Kendall the consumption appears to be much less than the synthectics.