The TBN level of motor oil in service drops predictable with mileage. But does it also drop with age in infrequently used vehicles? I'm not talking about short trips where the oil doesn't get up to temperature and is contaminated by water, fuel etc. I'm referring to special interest vehicles of one sort or another that are driven infrequently but when they are driven the oil is brought fully up to temperature before the engine is shut down. Annual mileage may not exceed 250 to 3000 miles per year. Does the TBN level start to decline regardless. I've seen UOA of high mileage OCI's safely exceeding two years, but what about 3, 4 or 5 years?