"I read once that if the base number (BN) falls to 50 percent of the value of new oil, that it would be time to change it. Is this accurate for automotive gas engines? If so, would a few ounces of fresh oil in a 4-quart reservoir bring the base number back to acceptable levels? Note: Other parameters on the oil analysis are acceptable."
Base number testing, which measures the reserve alkalinity of the lubricant, is primarily applied to engine crankcase applications. It is the ability to neutralize acids created during the normal combustion process of an internal combustion engine.
The base number, which formerly was known as the total base number, can be defined as the quantity of acid, expressed in terms of potassium hydroxide, that is required to neutralize all basic constituents present in 1 gram of sample.
Once the base number reaches 50 percent of the starting value for that lubricant (specifically that batch of lubricant vs. manufacturer spec sheets), a cautionary alarm is generally broken. As the base number decreases over time, you can expect the rate of change to increase. This cautionary alarm gives you time to prepare for that increased rate of degradation.
Once the base number reaches a value of 2 or less, it is time to completely replace the lubricant. Regarding adding a few ounces to a 4-quart system, this can be confirmed only with oil analysis. You are basically sweetening the sump with fresh oil.
I assume by "a few ounces" you are implying a sample bottle or two worth of oil (a typical sample jar is 4 to 6 ounces). This likely would not result in enough of an increase of the base number to remove the lubricant out of an alarm status. With a sweetening of the sump, it is generally recommended to replace at least 25 percent of the lubricant. This depends on several factors, with the severity of the lubricant being the primary factor.
Less than a 25-percent volume would probably not yield results worthy of the time put forth to do so. If you look at equal units of measure, you likely would have a better understanding of this. You are thinking of adding a few ounces to a system that contains 128 ounces. You would still be able to drive down the road, but your protection would be gone as well.