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All I can say is, I've been using Shell Rotella T 15w40 for the last 4 years in my Universal 5424, and the previous owner used it for about 10. I buy it from Walmart.

If you really want 10w40, Shell make Rotella T5, a more expensive synthetic blend, in 10w40. I can see the advantage being easier starting on very cold days, and you're in a colder climate than me.

Both are well-respected diesel oils.

Interestingly, I just fixed a noisy tappet in my Land Rover V8, with Rotella T. Took about 500 miles to work its magic.
 

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I do think it's a good idea to use a diesel-focused oil like Delo or Rotella T, not just any 10w40. They have more of the detergents and surfactants to keep all that soot in solution. They also clean up a dirty engine, given time.
 

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"They start at lower vis and it rises as the oil heats. How? With chemical additives. Essentially, you are buying 10 or 15 w oil that has an additive that will increase viscosity with heat. Sort of. "

Actually, the viscosity of a multigrade oil does fall off rapidly with temperature, it just doesn't do so as quickly as a with a single weight oil.

Take a look at this table for Mobil 1 0w40 (just a random example). Click on "tech details".

http://www.mobiloil.com/USA-English/MotorOil/Oils/Mobil_1_0W-40.aspx#

The viscosity is 75 cst at 40C, and 13.5 cst at 100C. Over 5 times thinner at 100C, than at 40.

A single weight oil would thin much more drastically, that's all.

All the viscosity modifiers can do is slow the rate of thinning with rising temperature.
 

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A caution note about using synthetics in an older diesel engine..

Using synthetic oils in older engines
I never suggested using a synthetic, I suggested a synthetic blend. There isn't enough synthetic in a blend for the engine to care at all, but it's the only way to get a 10w40 without use of lots of viscosity modifier.
 
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