TAN is used to measure the total acidity of the oil, every lubricant has an neutralizing agent to react with this acidity and that is being measured through TBN. TAN will determine if the oil needs to be change or not, An oil analysis test which will determine the remaining useful life of oil is the RULER, but this is somewhat expensive, hence an alternative will be the TAN. TAN should always be less than 1. A high reading on TAN means that additives used to neutralized the acid had already been depleted and the oil must already be change.
Many people believe that you have to change oil regularly. The only indicator that would tell you that you need to change your oil is the TAN (Total Acid Number). If your TAN is greater than 2 then you had been shot and your oil
needs to be changed. If TAN is under one , then the oil is in excellent condition unless you have an unusual amount of water in the oil. If your oil has a high amount of particulates don’t change it if the TAN number is good, filter it.