I recognize that science and facts carry very little weight in oil discussions but that isn't going to stop me from injecting some. Here is how the chemical engineer would analyze this. First think of time as miles. Take the volume of the crankcase (7.5 qt) and divide my the rate of oil addition (1qt/800 miles) to get a space time or time constant of 6000 miles. Assume (you decide if this is a good assumption) that the rate of contamintation in the oil is constant (independant of the concentration of contaminants already inthe oil) and that the critical concentration is reached at the factory recommended change frequency (7500 mi.). This assumption means that the rate of oil contamination is 1/7500 miles. Mulitplying the time constant by the rate we get 0.8. This means that if you never change the oil and continue to add 1 qt / 800 miles the oil will reach a constant contamination level equal to 80% of the contamination level at 7500 miles under a no leak/normal change program.

If this were a diesel with a 5000 mile change frequency the oil would reach a steady state contamination level 20% higher than that at the recommended change time. I won't go into the math here but to keep from exceeding the critical concentration one would have to change to oil at 9000 miles and then start over.

Caveat - It's been 30 years since I took this course. Perhaps someone will check my math.

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Tom Savage
Vienna, WV
1984 300D Euro
1995 S320