In the past most fuel dilution issues of crankcase oil in turbo charged diesel engines meant there was a mechanical problem of some kind.
For 2007 and newer Ford 6.4L, Dodge 6.7L and Gm/GMC 6.6L turbo diesel engines, the emission control systems sometimes are the culprit, as these have been installed to meet more stringent environmental protection agency requirements pertaining to soot.
Not everyone will experience dilution problems, as it will depend largely on how you operate your vehicle. Post 2007 turbo diesels are equipped with a Diesel particulate filter as mandated by the environmental protection agency. These filters reduce the soot emitted by as much as 85%. After so long, these filters have to go through a cleaning process called regeneration. Regeneration can occur during operation when exhaust gas temperatures reach 1,100 degrees. This happens regularly when the engines are worked as intended, such as towing and hauling and is called passive regeneration. It is when these engines are operated around town with no load is when temperatures are to low to passively burn off the soot.
When temperatures do not reach 1,100 degrees, an active regeneration must take place. Two systems are used. In-stream injection and In-cylinder post injection. What happens is raw diesel fuel is introduced into the diesel particulate filter increasing the internal temperature to burn off the soot that has accumulated.
With In-stream injection, the fuel is introduced through a separate injector either behind the turbo charger or in the exhaust stream just prior to the particulate filter. This type of active regeneration is largely used on medium to heavy-duty diesel engines. In-stream does not contribute to fuel dilution.
In cylinder post injection has been adopted by the manufacturers of lighter turbo diesel trucks, mainly because of the lower cost. With this system, the diesel fuel is injected directly into the cylinders with the present injectors but on the exhaust stoke as well as the compression stroke, sending the unburned fuel through the exhaust ports to the particulate filter and be burned that way. This system works well at delivering the fuel to the particulate filter but has the drawback of promoting fuel dilution of the crankcase oil. As the fuel is injected on the exhaust stroke, fuel is able to pass by piston rings and wash down the cylinder walls as well, entering the crankcase and diluting the oil. Diesel fuel is a solvent and can reduce oil viscosity and film strength of the oil. An increase in oxidation can also occur.
At this point, there is no correlation between fuel dilution and abnormal engine wear. This is a rather new issue and still being studied. Condemnation limits have been set at 5%. Oil analysis samples have shown 7 to 10% dilution rates.
If you have one of these turbo diesels and work it hard, active regeneration will be at a minimum, as will the chance of higher limits of fuel dilution. It will be beneficial for you to have your oil analyzed periodically to see what the saturation level may be in your oil. It would also be a good idea to use premium oil that can better withstand the effects of fuel dilution.
In closing, for the 2011 Ford and General Motors are changing to a system called selective catalytic reduction, eliminating In-cylinder post injection. The Dodge Cummins turbo charged pickups will continue to use In-cylinder Injection for the upcoming year.