The Total Base Number (TBN) is a measure of the engines oil's ability to neutralize any acid forming material that is formed during the burning stage of the fuel.
Usually the higher the TBN, means it will fight acids better. It sometimes happens like that and sometimes not. It depends how good the additive pkg is. With today’s Epa driving the level of the oils, the oils need to be able to take “more junk” in the oil. As I noted in older posts, the TBN will get depleted overtime. The “trick” the oil people are trying to do is slow the rate of change. You could have a TBN of like 13 but the additive pkg is poor and it drops like a rock. You could have a 11.1 TBN and it drops more linear and slower.
As Dargo stated wonderfully, /forums/images/graemlins/grin.gif, it is not 100% that the oil is great with a high TBN, but I would rather have a high or higher TBN then some other oil starting at lets say 6 or 8.
Most times if the TBN is high along with a high ZDDP pkg, you are good to go.
Also the ash comes into play, not integral but some oils have ash. I would not want ash in my oil for reasons stated above. But if your Noack Volatility (NV) is high, means you will use more oil. If you use more oil, the ash level will rise in % with a high NV since the ash does not disappear. This NV I consider it like evaporation. So unless you top off ASAP, I would rather have zero ash in my oil.
Think of it like this, I have a salt-water fish tank and I got some evaporation due to no tank tops (which I want for gas exchange from DSB). If I never top off with new RO/DI water the salinity or SG (salt level) will constantly rise. The salt leve lwill rise since the water evaps but the salt does not. So I need to add water to bring down the SG or Salinity.
It is the same for oil when worrying about ash. If the NV is high you use more oil and the ASH will rise. It your ash rises it will/might cause slugging and oxidation unless you add more oil to bring the levels back down. So pick a oil with a low NV etc.
I could type for hours here so I will stop. I want to fly my heli in the snow!