TBN vs. TAN?

[cp3]

I'm thinking about doing a used oil analysis on the Saturn as it has been on a strict 5000km OCI and want to see if I can extend it a bit. I plan to use Toromout-Catapilar for the test and they apparently include TBN and TAN in there reports. I have read some stuff about using a TBN/TAN comparison (when TBN is equal to TAN the oil is used up) to determine the life cycle of the oil. I rarely see anything on these forums regarding TAN though.
I am wondering if that is because it is not in most used oil analysis reports or it is just not deemed to be a solid method.

I also believe I read on here somewhere that it's best to wait until the second OCI on a particular oil before doing a used oil analysis. Is that the popular theory? I ask because it has been run on GTX for a few years, has MaxLife now and will be getting Valvoline for the next 4 or 5 at least.

 

[JAG]

There is no 1 hard and fast rule for TBN/TAN relationship that applies to all engines and all oils. It is a difficult thing to determine the max. OCI to follow. used oil analysis will help guide you but also keep an eye on the cleanliness of your engine by peeking inside of it wherever/however you can. Some engines will varnish/sludge even when the used oil analysis look "not bad". Others will stay clean even when the oils are run to their limits. Knowing your engines trait in that regard is important.

I could provide some links for further reading but even better is for you to Google this phrase and read the links that look helpful: TAN TBN condemnation limit

 

[INDYMAC]

I think a good starting point would be to get a VOA of the oil you plan to use first. And then do periodic used oil analysis throughout the OCI while watching the rate of increase in TAN, and decrease in TBN. When you start to see the rate for either start to get worse (increase rapidly), time to change the oil.

That's free advice. If you want professional advice from someone who actually knows what they are talking about, call Terry Dyson.