"I am looking at formulating a long-drain diesel engine oil predominantly for U.S. engines but without the constraint of CJ-4. What are your views on the relative merits of overdosing a CJ-4 formulation vs. just boosting the detergent inhibitor?"

When considering formulating your own diesel engine oil, there are many factors that come into play. First, understand that the CJ-4 formulation places limits on additive levels to protect after-treatment emission-control equipment on 2007 and newer engines. These engines are designed to conform to CJ-4 classifications, which still meet all the criteria of older diesel engines using CI-4 and CI-4 Plus. The CJ-4 classification also sets limits to comply with the Environmental Protection Agency (EPA) standards for using new ultra-low sulfur diesel (ULSD) for on-highway diesel engines.

Starting in January 2007, the EPA placed tighter limits on the exhaust emissions of diesel engines. To abide by these limits, diesel manufacturers began adding after-treatment emission-control equipment such as catalytic diesel particulate filters and oxidation catalysts. The CJ-4 classification was formulated to protect these pieces of equipment.

CJ-4 formulations are intended to protect vital internal engine components while not poisoning the catalysis or blocking the particulate filter. EPA regulations mandate that diesel particulate filters operate for a minimum of 150,000 miles before servicing. Changing the additive formula in the CJ-4 formulation could greatly affect the particulate filters, requiring earlier servicing or poisoning of the catalysis.

Ultra-low sulfur diesel has an EPA mandate of 15 parts per million of sulfur. What does this mean in terms of engine reliability and running conditions? ULSD has a lower heating value, resulting in worse fuel mileage. Also, due to lack of lubricity, it can cause premature injector wear. CJ-4 was designed for use with ULSD to give added protection from sooting and higher cylinder temperatures.

When changing an oil formulation, you hope to enhance a certain property. However, what usually happens is that by improving one oil property, you alter other properties in a negative way. Overboosting the detergent inhibitor can have an impact on how the oil disperses deposits or controls friction and wear.  

Keep in mind that when formulating your own oil or brewing additives into an already marketed formulation, there is always a give and take. When doing so, have your oil tested against the corresponding ASTM test methods and EPA regulations so no harm is done to the engine or the emission-control devices.