Here are the specific reasons not to use Mobil 1 or any other automotive oil in a diesel:
1. Unlike gasoline, diesel fuel contains lots of sulfur (typically 330-500 ppm), which forms acids in the presence of moisture. Moisture is unavoidable since water is a product of combustion. Diesel oils contain detergents that are “overbased” by adding excess calcium or magnesium so that they have a high capacity to neutralize acid. This overbasing is measured during oil analysis as the total base number or TBN. Diesel oils such as Delvac 1 have a TBN of at least 11, whereas the TBN of fresh Mobil 1 is around 7-8. TBN decreases as the additives in the oil are used up with mileage and time. Many operators replace oil once the TBN reaches 7 which is where Mobil 1 starts out.
2. Gasoline burns efficiently and produces almost negligible amounts of incomplete combustion products (soot). Diesel fuel, on the other hand, always produces soot. Most soot goes out the exhaust, but some gets scraped back into the crankcase by the piston rings. Soot starts out as individual particles that average about 1 micron in diameter. These are too small to cause significant abrasive wear. But soot tends to aggregate into larger particles that can cause engine wear. The larger particles interfere with the hydrodynamic lubrication properties of the oil and eventually settle out as sludge that can block oil passages. So diesel oils must contain high levels of dispersant additives that coat the soot particles and prevent them from aggregating and hold them in solution. Delvac 1 can hold soot up until 7% of its volume and still function as a good lubricant. Automotive oils like Mobil 1 are not designed to hold soot in suspension.
3. The conditions in a diesel engine operating at highway speeds promote oxidation of oil components. Premium oils designed for diesel engines typically contain higher levels of oxidation inhibitors than automotive oils.
4. The API limits the amounts of certain additive components in automotive oils in order to extend the functional lifetime of catalytic converters. The limits are slightly higher for diesel engines. This allow manufacturers to include higher levels of anti-wear compounds in diesel oils. The greater amount of these compounds in diesel oil provide additional protection under extreme conditions where metal to metal contact can occur.
Make sure than any oil used in a diesel engine meets CH-4 standards. Using Mobil 1 or any other automotive oil in a diesel won’t cause immediate damage, but the long term effects are cumulative. It’s sort of like smoking or eating too much fat when you are young. You don’t notice the effects until you are 50 or so. The Cummins is so well made that you probably wouldn’t notice the effects of using a good synthetic automotive oil like Mobil 1 until 80-100K miles. But by then it would be too late and you will have shortened the life span of an engine that is designed to last 400K miles.
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