Dear Ole Mechanic;

I have a 1980 Mercedes 300 SD.  The manual says to change the oil every 5,000 miles.  I’ve heard that the new low-sulfur diesel fuel does not have the same lubricating ability as the previous diesel. What effect does this have on my oil changes. I've been using synthetic oil in my car.Low-Sulfur Diesel Fuel

What would you recommend and at how many miles.

Diesel Bill W


Dear Diesel Bill;

Unfortunately, the diesel-fuel lubricating ability that you heard about has nothing to do with which oil you use or how often you should change your oil.  It is a ‘Good News/ Bad News’ thing.

The good news is that less sulfur in the fuel that you burn means less oil contamination.  You could actually extend your oil change interval some.  However, the 5,000 mile change-interval will still work fine.  When the fuel burns in the combustion chamber,there will always be a small amount that blows by the piston rings--primarily through the piston ring end gaps--but some does get past the rings themselves.  These combustion byproducts are what contaminate and dilute the oil in the crankcase and iareone of the main reasons why the oil needs to be changed.  With less sulfur in the fuel, there will be less sulfur getting into the oil as a contaminate.  This is a very good thing, as a brief chemistry lesson will explain. 

What we want to burn in our current gasoline and diesel engines are HydroCarbons.  If that was all that was in our fuel, the exhaust would consist of Hydrogen plus Oxygen (from the air), which would give us H2O--or water (vapor due to the heat of combustion)--and Carbon plus Oxygen giving CO2 or carbon dioxide out of the tailpipe.  Unfortunately, there are many other things in crude oil that are difficult to refine out when making gasoline or diesel fuel.  One of these items is sulfur.  If you add Sulfur to water vapor (H2O) in a hot environment--like a combustion chamber--you can get H2SO4--commonly known as sulfuric acid.  Sulfuric acid is extremely corrosive--even to the iron inside an engine!  Fortunately, there was not much sulfur in our past fuels, and now there will be even less.  That is the good news--and now for the not so good news. 

The reason for the concern about the lubricating ability of diesel fuel is the fact that the fuel must lubricate the internal moving parts of the fuel injection pump and fuel injectors.  The injector pump is NOT lubricated by the engine oil.  As the injection pump must build up around 2000 PSI (pounds per square inch) of pressure and liquids are almost uncompressible.  That means that there are extreme pressure points inside the pump that must be lubricated by the diesel fuel.  This is where the loss of lubricating abilities may cause problems.  If there is a wear problem in the pump it can be very expensive as the clearances in some of the moving components are measured in one-hundred-thousands of an inch.  We are talking about a clearance of one tenth of the thickness of a human hair--a fine human hair, at that.  Now I did say “. . . may cause problems” because I have not seen any reports of problems, but there may be some that I am not aware of. 

One thing in your favor is that Mercedes has been making diesel engines for world wide use for many years and for many types of fuel.  A call to a Mercedes dealer’s service department or to a factory customer-assistance-representative might get you more up-todate information than I have.  If it were my car I would probably not worry about it and just continue to buy a brand name of fuel.  If there were a concern raised by the dealer or customer help line I would probably just dump a quart of non-synthetic motor oil in the fuel tank when the tank is filled.  A little more expensive peac- of-mind would be the diesel fuel additive of your choice.

Herr Professor Nuzanbolts