OK guys - if this one is obvious please feel free to blast away at me:

I have a great commute. A third-gear, up-and-over, high-speed mountain road commute. It's 15 miles each way and in third gear the car stays between 3300-4900 rpm the entire way (about 50-70mph). It's HUGE fun and the car does just fine. It doesn't even eat much oil. In fact, if I don't have other long-distance trips between changes I can make it 2.5k miles w/o adding oil.

HOWEVER, I just completed a cannonball-style run down south to see family and over 900 short miles it burned through 1.5 quarts of oil. I spent nearly the entire time bouncing between 85-105+mph, but actual engine speed (in fourth gear this time) was about the same range as my regular daily commute. Keep in mind this kind of consumption is not abnormal for the car - I've been noticing disparity on my high-speed, extended travels since 60k miles or so (currently 94k), but have just now decided to try and figure it out.

So what gives? Is there a correlation between higher actual speed (versus engine speed) and oil consumption? I noticed that I was waaaay deeper into the go pedal on the long trip (at high speeds) than daily when spinning over the hill and I'm thinking that fuel/combustion load might have something to do with it?

Ideas?

Mak,
I believe it does.Last May,I drove from
Chicago to Maryland,averaging 60-65 mph.,
and once in a while a blast of 75-80 mph.
That is about 740 mi.and my car consumed
a full quart.The manual stated that,
the car will consume oil if you drive
agressively.However,on normal driving,
my oil consumption exellent.

I have lousy oil consumption but that is besides the point. One reason for your sudden increase in oil consumption is this. Over the course of normal driving, especially short trips (15 miles, engine oil does not get hot enough to totally burn off all of the contaminants (acids, condensation, carbon, etc) that are a byproduct of internal combustion. This contaminants build up in the oil and can actually contribute to an increase in the oil level. So even if your car is using oil at acceptable levels these contaminants can make it look like your car is using less oil than it really is. When you go on long trips the oil is given time to get really hot and stay really hot long enough to burn all of these things out of the oil. If you take a long trip towards the end of your oil change cycle you will notice a bigger burn off. If you put fresh oil in just before your long trip your oil consumption will be much less (assuming no leaks, or abnormal consumption). In addition to the contaminants burning off, the oil itself can burn off, this is most noticeable on long trips. Try using an oil with a higher flash point for long trips.

If you are curious, change your oil, then take you normal trips (15 miles each way until you reach about 3000 miles, then take a 700-1000 mile trip and measure your consumption. Then change your oil and drive back home, measuring oil consumption again. I have done this several times and I always use much less oil on the way home.

__________________
Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

Hmm - oil getting thinner and burning off faster as it gets wears out. Sounds too logical to be true. Thanks Jason!

While on the same issue, I have a related question - should one change oil less often if one does more highway driving? I change it religiously after 3000 miles when I have been driving in the city/suburbs. But this time, out of the 3000, 2000 was highway (long stretches) - should I still change it at 3000, or go till say 4000? Basically, does the oil quality deteriorate as much by highway driving as by city driving?

thanks!

Rags
85 190E 2.3 (119K)

Highway driving is less stressful on the engine (unless you are pulling a three hour stretch through the mountains of West Virginia at 80mph) and the oil has time to get hot and eliminate harmful contaminents it carries away. I would say that you could safely extend your change interval if you are doing mostly highway driving. It would be interesting to hear from anyone with FSS on this subject. Does the computer agree with me? Either way it can only do more good than harm to stick to your current schedule for oil changes. Change it hot and change it often.

__________________
Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired