Correct me if I'm wrong, but there shouldn't be any issue with running 5w40 year round. If it acts like 40 weight at operating temp, that means its thinner at cold than a 15 weight. So when you start the engine the oil can get up top quicker and easier in the summer and the winter.
5w40 is fine year round....a lot of companies are doing research now for diesel oils as thin as 5w30 and 0w40.....the oil companies are trying to get fuel milage increases by using thinner oils that has the same lubricating properties and protection as heavy weight oils....they have already figured this out for the 5w40 diesel oils.....just remember that any oil that is is rated diesel specific has been ASTM tested in heavy duty Mack, Cummins, CAT and Detroit engines so running it in your passenger truck which doesn't see nearly the operating conditions as one of these big brute engines is more than enough protection...
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