Side benefit of Oil Leak -- Perpetual Oil Changes?

[Benzwood]

My W124 has the infamous head gasket leak, plus (I'm guessing) some valve guide issues, with the combined effect that I have to add a quart of oil every 700-1000 miles.

I will fix this one day when I can get the car to someone I can trust to do the work correctly (local independent definitely does NOT fit that description), but in the meantime I'm curious...

Do I need to change my oil if I'm refreshing it so frequently? Obviously I would still need to change the filter, but that's an easy no-mess job.

Just a hypothetical question at this point, don't murder me.

[Jackd]

Yes, you need to change your oil to get rid of all of the contaminants that are not absorbed by the oil filter.
Acids, small particles not trapped by the filter, ash, sooth, silica, metal shavings, oxidize oil and about 12 other "bad" things.
JackD

[Benzwood]

I realize the oil is trapping contaminants not caught by the filter... but my car is eating oil.

So wouldn't those suspended contaminants be leaking out and/or burning up along with the missing oil?

[Mike Murrell]

Originally posted by Benzwood
I realize the oil is trapping contaminants not caught by the filter... but my car is eating oil.

So wouldn't those suspended contaminants be leaking out and/or burning up along with the missing oil?

If ALL of the oil were leaking out, the answer to this would be yes, but it isn't.

Change your oil & filter according to schedule(not the one the mfg. provides).

[Hatterasguy]

Try it see if it works. Then report back to us.

[Ali Al-Chalabi]

I would strongly recommend against that practice.

[benzthere]

To test your hypothesis, send a sample of your oil to Blackstone Labs. It is amazing what they can tell you.

http://www.blackstone-labs.com

[Duke2.6]

Originally posted by Benzwood
I realize the oil is trapping contaminants not caught by the filter... but my car is eating oil.

A quart every 700-1000 is hardly "eating oil". When it gets down to a quart every 100-200, THEN it's eating oil.

Maintain the recommended oil change intervals.

A thousand miles of fuel costs costs about a hundred bucks. The quart of oil it consumes in that period is about equal to a cup of coffee at Starbucks. Hardly a big deal!

Duke

[Benzwood]

Ok, ok, I'll change my damn oil. Lousy high-maintenance Benzes.

That Blackstone test looks kind of cool, has anyone ever sent a sample from a 1986-1992 300E to them?

I might do it just for kicks if the numbers could be meaningfully compared to someone else's.

I see from their web site they give you a "universal" average of their testing for your car make/model, but I don't know what they consider a "model".

[william rogers]

It's probably within factory alowed oil usage.changing oil on an MB is very easy compared to most cars at least on 126's and it cost very little...........

William Rogers........

[savagetom]

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.

The math in the last post works, but ONLY if the oil is lost due to leakage, NOT by worn cylinders or rings.

If the engine were worn such that was BURNING the oil due to blowby conditions, you actually should change oil MORE often. This is because there is a serious amount of combustion byproduct contamination making its way past the rings and into the crankcase as well as through the crankcase vent.

Good luck,

[Benzwood]

Thanks all for the advice.

I changed my oil myself despite the siren call of Jiffy Lube, hard to resist when worming around under my car on my midwest-winter mud/snow/grime infested garage floor.

In the process I found out that the independent Euro-mechanic who last changed the oil apparently likes power tools. I was afraid the drain plug wasn't going to come free without damage. Even the air filter nuts were absurdly tight! Yet another reason not to go back to them.

It also gave me the chance to remove about 30 pounds of grime from my lower engine cover. I installed that a few months ago but it seems to do a better job of retaining grime than keeping it out.

I may invest in one of those fancy Fumato oil drain valves, the kind with a nipple for a tube. Seems like that would be a way of avoiding a lot of mess without resorting to the Topsider route -- could drain right into an empty oil container for recycling.

Maybe also cut a hole or flap in my engine cover to allow draining the oil without removing the cover -- anyone done that?