I personally run 15w-40 t4 dino in the summer and right now I have 5w-40 t6 synthetic and a quart of Marvel Mystery in for the winter. Nearly 1000 miles and absolutely no drop in oil level. If you have an average winter temp around <40*f the 5w-40 is great stuff, really helps get things lubricated and turning, and runs nicely all the way up to temp. Wouldn’t hesitate to use it in mild summer climates as well. I’m sure the 15w-40 t6 is great stuff as well.
As far as turning it black, I think that depends on a whole lot more than just which oil you use. A lot of oil likes to linger in the valvetrain, oil pump, oil cooler, etc. so as soon as you churn it up with any new oil you’re gonna have oil that appears black, but you can tell if it’s saturated or not, it’ll get a whole lot darker and saturated towards the end of your oil change interval. Also, the more worn your engine is the more soot it will produce, making any oil you put in it turn black quickly.
Diesels, by nature produce a lot of soot, and if the engine has never run synthetic or oil additive, or been cleaned/rebuilt, or neglected by running saturated oil in it for too long, theres a good chance there is soot buildup and as soon as you run synthetic for the first time its going to start breaking down the sludge and deposits, which will turn the oil black rather quickly, potentially faster than dino oil the first time you use it. But as Jasper said, if you keep running synthetic every interval, it’ll get cleaner and cleaner over time. This is why guys complain about synthetic making their rigs leak, because it cleans out the soot buildup left by previous, sooty oil in the voids of bad gaskets and seals and gives the illusion that their truck doesn’t have leaks until they ran synthetic, when in reality, those leaks were already present, just clogged with soot/sludge.
My recommendation is to not worry that much about black oil, 99% of the time your oil is going to be some shade of black, unless your motor is fresh, and even then, its still a diesel, it’s gonna have black blood. If you have the money, synthetic all the way! Cleans and lubricates nicely, and can hold soot in suspension better than dino oil without breaking down, giving you better longevity, but I wouldn’t say it’s significant. Definitely don’t expect to go 20,000 miles on synthetic in these things, just replace your oil at the same 3000-5000 mile interval. Some guys do oil analysis and can get away with it, but for the average Joe, just do your engine a favor and change the oil. But, just be prepared that if you use synthetic you’ll be cleaning the motor, which is great and you should, but if you have any failing gaskets it will find them eventually. It won’t ruin gaskets and seals, but just unmask bad ones.
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