Originally Posted by sobiloff
The problem is Ford's design for the soot filter cleaning cycle. Other manufacturers use a dedicated fuel injector just in front of the filter to pump extra fuel into the exhaust to increase filter temps and clean the soot off the filter. Ford uses the existing injectors on the left bank to inject diesel during the exhaust stroke. This also works to add fuel to the exhaust system and increase exhaust system temps to clean the filter, but since there isn't an ignition event happening when they do this the rings aren't being pressed against the cylinder walls and some of that unburnt fuel makes it past the rings and into the crankcase. For some chemical reason that's beyond my expertise, biodiesel is more likely to get past the rings than petro diesel. This leads to fuel dilution of the oil, hence Ford's requirement that you use a 40-weight oil if you're going to use B20.
My 2007.5 Chevrolet Duramax cleaned its DPF using this method and Chevrolet has since moved to a dedicated ignition source to prevent furl dilution.
I did oil samples every 7,000 miles and never saw high levels of fuel in my oil. There was some, but the lab always said "continue service".
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