Generally I view condemnation of the lube to occur when the wear rates of metals would excalate past a "normal" trend. To do so, you have to know the macro data trending of your equipment, and also it helps to know your micro data trends.
The inputs you inquire about (fuel and soot), are to be looked at in context of the overall equipment health history. If your engine routinely turns in good UOAs even though the fuel is at 4% or less, then there's no reason to condemn the lube. If 3% fuel often shows wear metal trends typically escalating in an agressive manner, then that would be a trigger.
The key to understand is that if the inputs are changing, it's only a trigger to continue monitoring, albiet a bit closer (UOA more often, perhaps). If the inputs are changing in an undesirable manner AND the wear rates are changing in an undesirable manner, it's time to plan an OCI. But even then, you want to understand the RATE at which the change is occuring. If the wear rate of Fe is typically around 2.2ppm/1k miles for your engine, and it rises up to 2.6ppm/1k miles, that would not be a panic moment. However, if the wear rate doubled to 4.4ppm/1k miles, then I'd OCI.
We've seen many, many folks do an OCI when their base/acid inversion happens; that's been a typical historical trigger for many folks. However, when the metal wear rates are either steady or even dropping, why change oil simply because the TAN usurped the TBN?
Inputs should not be, in and of themselves, a trigger for an OCI. They should be a trigger to pay closer attention to the wear rates of the metals.
NOTE: I will conceed that if an input goes way, way out of normal bounds very quickly, an OCI is warranted immediately, but one should ALSO go find out why the input spiked (leaking injector, void in the air filter system, coolant loss that is not ending up on the floor but inside the crankcase, missing cover or plug, etc ....)
