If your using over 5% you will have decreased mileage for oil changes. Its states it in the owners manual. I believe its due to the oil dilution from the bio diesel. The benefits from a small percentage of bio diesel should offset any negatives associated with the fuel dilution. If it is a concern you can always do oil analysis to check your oil. I did an oil analysis at least once per year on my Duramax.
I know Capt said he doesn't like it... but unlike ethanol... it's not a huge hit to the MPG's. But it has the benefit of being an EXCELLENT lubricant for the injection pump and injectors. Because of that... I like to see a minimum of 2%.
Here in PA... the State Gov supplements some Bio plants... so we get 2% to 5% in all of our diesel.
I use it all the time because I'm cheap. Seems I get a 5 cent/gallon fuel discount at the Murphy stations with a WalMart credit card. Most always that's the only fuel I use. Seems that the major truck stops have prices 10 cents or more higher per gallon than where I buy so I don't buy there. Just figured biodiesel was a dilution product that decreased fuel mileage. If not, then it's a good thing. I don't use fuel additives and now, without knowing it, I guess I do.
Ill say this about bio-diesel, don't use it in the winter (if you live in a cold state) it will start to gel at around 35 degrees farenheit. Even a small percentage can cause your fuel filter to plug up. If you can't avoid it, be sure to use additive. Otherwise there is no harm in running up to 20%. If the truck is rated for it, it should be fine. Biodiesel also absorbs water (similar to ethanol) so it may be a good idea to drain your water separator every once in a while. some have experimented with running over 20% on other common rail diesels, but i would stay away from "experimentation" on such a new platform. Also the CP4.2 injection pump that these, the 6.7 power stroke and the LML duramax come with are known to be inherently weaker compared to the CP3 found on cummins engines and older duramaxes, and prone to more failure, and when/if they do fail they will send metal downstream to the injectors and trash those as well. So i would say running some bio-diesel to help with lubricity is a good thing, but be cautious of the water intrusion. Also these pumps are very sensitive to gasoline, so make sure if you put any in by mistake to drop your tank and get it throughly cleaned. the last thing anyone wants is a 10k repair bill for an entire fuel system to be replaced and not covered by warranty because there was "water in the fuel" or you filled up with gasoline by mistake. Just a word of warning.
Since I've got 6 -8 weeks to kill waiting for mine to arrive, I read the owner's manual, user's manual, and the diesel supplement (downloaded from the Ram website). After reading all the warnings and precautions and limitations regarding Bio diesel in these manuals, I would never use it it my truck. In an emergency, I might put a small amount of <5% bio diesel in the tank. The concern seems to be oil dilution/contamination that mandates more frequent oil changes. Since an oil change is about $125, i don't want to change it any more often that required. More frequent oil changes would probably offset any savings in fuel costs as well. Just my opinion based on what Ram says in their product information.
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