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I agree that synthetic is a lot more expensive if you are going to change it every 5K, but what is more expensive... changing your oil & filter or taking a sample & doing an oil analysis? An oil analysis will give you the facts on your oil (& filter) and is certainly a much more scientific & cost effective way of determining when your oil needs to be changed. That is why railways, airlines & trucking companies have been doing it longer than I've been alive.
True - I wouldn't bother doing an oil analysis on a little gas engine with 4 qts of oil... it would be cheaper to change it than to test it, but there certainly is a cost effectiveness when a crankcase holds 3 gals .....
How about over 65000 miles on a 7.3 without an oil change? How about over 400,000 on a Mack without a change? (OK, no HEUI injectors there).
Of course, cheaper oils that are prone to shearing (thank you HEUI injectors) would not deliver results like that.
Dino is still cheaper?
With filter changes (as specified by OA), normal consumption (& leaks), there are small amounts of new oil (& additives) being added over the course of time.
As far as the original question, the 15W-40 HD Diesel & Marine oil has the highest TBN, so it is still probably the better choice.
Your mileage may vary.