I vote 15W-40, unless you live somewhere where it gets brutally cold. Use a good quality, diesel-rated, oil like Amsoil or Rotella. I just bought Rotella for $8.88 a gallon @ Wally's World. Can't beat that.
Well I use 15W40 Rotella, Baldwin Filter and a quart of Lucas oil additive in mine. I go oil change to oil change without ever having to add a drop of oil to it...they say its because of the Lucas. The truck has 234,000 miles on it!
I have been using 5w30 Amsoil HDD diesel oil year round for the past 4 years and I still get 15-18 city and 18-25 hwy, thats almost 5 years with no oil change,40,000+ miles just add oil and change filters every year, I used the AME 15w40 oil initially but wanted better milage and I got it, here is my last oil sample. www.voogru.com/images/oil2.jpg
i was using Amsoil 15w 40, now i'm using 5w 30, i cant lie, i switch back and forth between amsoil and diesel kleen in my fuel. I dont use lucas at all, its well documented that it is not beneficial. Rottella's ok oil, Amsoil is a far superior lubricant in my opinion. let me know if i can help.
I will avoid the brand war over which oil is better. Use what you want and if you travel what you can find on the shelf at random truck stops.....just a thought.
I will reiterate that unless you live where temps get cold (well below freezing) you will be fine with 15W-40. The first number signifies what the viscosity of the oil is when cold and the second number is the viscosity when its warm. So as the oil warms its viscosity changes. If its not cold then the lighter weight oil isnt necessary.
That was a basic not indepth look at oil. Not the subject matter expert but that should suffice.
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