IMO, the answer is roughly the same as what I posted in the Group V oil change discussion.
Quote:
TBN retention and fuel/moisture dilution will be as telling as anything else, and are often another reason why the more expensive fluids offer poor value proposition as majority content in oils. Add packs and a balanced lube design is more important. And good grp III oils will have similar oxidation performance as higher group lubes, so it is best to ensure that the only benefit, flow at ultra-low temperatures, is actually beneficial. If at your typical ambient temperatures, the viscosity is similar, then what's the point?
Some of this holds true. For vehicles that are primarily run for long, steady-state use, like over the road trucks, a shear stable lubricant with sufficient TBN may well be able to be bypassed, reduce soot loading and then just run for very long times.
For for passenger vehicles, what are the differences? First, most arent used on extremely long runs consistently. Sure, traveling salesmen and folks with really long commutes may be able to approximate this, but it just isnt the same as say, over the road truckers. As a result, the typical vehicle user has to worry a lot more about moisture and fuel in the oil as a means of degradation than a long-haul user necessarily needs to.
Sump capacity also has something to do with it. Large sumps with high levels of cooling can maintain temperature better, and thus should help reduce oxidation rate, which is chemically dependent but also dependent upon temperature. The larger sumps also have more active additive, more TBN buffering capability, etc.
Additives do get used up, VIIs get worn out, etc. All these things contribute to the need to refresh the oil. I do not buy 500k without needing to change the oil. There is bound to be some other aspect of the operation, either frequent filter changes, consumption, leakage, etc. that contributes to the replenishment of oil in the sump which keeps new additives coming in.
To me, the biggest benefit of bypass is to minimize soot loading. IMO the main determinant in many diesel engines has been when the soot loading was getting too high, not oxidative thickening, shear thinning, loss of adds or even necessarily loss of TBN or dilution of fuel/water. This is all especially the case for long-haul use which can be a fairly easy type of application (think highway cruising). If one can reduce much of the soot loading, then the time at which the oil must be replaced due to the dispersants being used up can extend out in time theoretically. My MB diesels all have a built in bypass filter in the oil filter itself, for this reason. But, newer ones dont have it in the filter. Why? Because more modern engines load soot so much slower than my older ones that it becomes a needless added cost, size and complexity. Remember, improving engine technology is a turnable knob in all this too!
But in the USA, most vehicles are gasoline operated, so soot isnt as great a concern, and most personal vehicles are governed by other issues associated with lube wear-out as I described before. So between the added cost, complexity, and other features of operation that have come to pass, I just dont see the need or ROI to be compelling.
