You should be using diesel rated synthetic like 5w-40. I'm using 5w-40 blue bottle Rotella as its the least expensive of the diesel sythetic 5-40s. The engine seems to quite out quickly in the cold weather and the cold flow characteristics are much better than the 15w-40. The 5-40 can be run in the summer to, but if you want to run a 15-40 you should still run synthetic.
Jason
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