At a full-service gas station yesterday, the kid accidentally filled my 88 300e with 87 octane gas. He was busy, and things got screwed up, so no big deal but I checked the fluid levels of everything before leaving the gas station and decided to test the performance of the car with this low grade gas. Performance wise, no difference at all and i guess as Ive got that crazy resister chip that retards the spark or whatever, no pinging.

The oil level was at 3/4 full when i left the gas station. 60 km later, the oil light comes on! I pull over, no sign of leakage, but the dip stick is below the minimum level; of course, i carry vital fluids in the car and top it off to full again....by morning, no sign of leakage, oil level at full...question - where did the 3/4 litre of oil go in 60 km?

I think its in the oil filter now, as burning "dirty" gas pollutes the oil, the filter does its job and soaks up the fouling agents but reduces the overall quantity of oil in the crank. I bought some octane booster and goosed the quality of the gas.

So, its off to the oil changers to drain the crap oil (after i burn off that tank of stink gas) and wonder for the sake of engine life why they sell such garbage. Im hoping it hasnt fouled the sparks etc, im assuming the oil is the vacuum cleaner of the engine.

Mercedes Fred,

While much of what you described as the function of the oil system is correct, it does collect impurities and then deposit them in the oil filter's filtering media, you cannot deposit an extra quart of volume there without making the filter housing larger. Loss of a quart or so of oil means you really lost it.

If you in fact lost a quart or so of oil in 60km, it either fell out on the road in a mightly splat for some reason, or it went out the usual way, which is through the engine and out the exhaust pipe. Spillage, leakage or some other external route would normally leave a few streaks of evidence.

I very much doubt a low octane fuel could inspire sudden and excessive oil consumption. In 60km you barely used a gallon of new fuel, in addition to what was in the filter, pumps and supply lines. In general for other than the octane of the fuels, the chemical composition of the fuel, including additives and grade of cleanliness is the similar for all grades at a given station. In some brands there may be more or less of certain "detergents" added in each grade, but the cheaper stuff is not something with chemically incompatible compounds in it that will attack your engine. So there is really nothing other than knock which should be affected, and your car apparently has the knock sensor to avoid this condition.

I would keep a close watch on oil consumption for a while, and if it returns to what it was before (I am presuming the new oil usage rate is much higher than before) I might repeat the experiment to see if the results noted in your post are repeatable. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Consider what Larry Bible has to say in this thread about the knock sensor issue. I agree with him. The gadget is there to thwart off a holed piston, but how long do you want to operate in this mode? The 103 motor in my car calls for 91 octane fuel. Mid-grade has been used & recommended here by some and I am not saying it or lesser grade fuel is inferior, but I personally am not going to try and 2nd guess the findings of MB engineers. I use 93 octane premium. I do realize, as you've stated that this was an unintended incident.

As for the sudden oil loss, I'd fully warm up the engine(15 or more miles of driving), park on a perfectly level surface and let it sit for several hours, then take a dip stick reading. I've seen all sorts of bizarre things happen in this area and have no valid explanation. If your engine would indeed have inhaled that much oil in that distance(seals/guides/rings), your plugs would be
drenched and the cars behavior would have reflected it. I suppose some sort of sudden external leakage could have occurred and I'd think that could be found with visual examination.

Octane requirements?

Your mishap filling your gas tank had no direct effect in emptying your oil sump.
There is no relationship whatsoever.
What could have happened (just guessing) is when you checked the oil at the service station, the engine had been off for several minutes with the result of draining the oil back to the oil sump. When you checked it again (60Km) later, just after shutting down the engine, the oil was still everywhere in the engine, and very little in the oil sump.
Regular grade gasoline (as compared to high octane gasoline) is not a a stink gas or a dirty gas. As a matter of fact, since high octane turn-around in a normal service station is something like 8 times slower than regular gas, you stand more chance of getting ''dirty'' high octane fuel.
Regular gasoline has not/will not cause any damage or other problem to your car, as long as it does not induce pinging.
JackD

Thanks for the observations, as I may have made a false connection between the coincidental ocurances of oil level and gas grade. The oil level has not changed since topping it up, and given the sudden onset of hot muggy weather in eastern ontario, then perhaps the engine is now having to deal with this environmental change which has an effect on oil.

If low octane gas is more quickly used than high octane (then by inference "cleaner"), would it be better to fill with low octane and use octane boosting products to get "premium" cleaner gas?

No. First of all, a good quality octane boost like Amsoil or NOS costs about $10. In addition once you add it to your tank, you cannot ensure perfect mixing like that from the pump. Some places will be really high in octance and others will be 87. Just stick w/ 91 octane, nothing less and nothing more. By the way, I ran my 89 300E on 87 gas for about 7 years and now I switched to 91 and I have no problems.