[DEnd]

Originally Posted by ME_Andy

Did you ever see anybody do this?

No but I did watch as an Acura threw a rod out of the block because its owner never changed the oil.

With newer Engines and oils 5-6K oil changes is about right. with better monitoring you can go longer

Acid in the oil is gonna get burned up by blowby, just like the oil does, so that's not a huge issue.

No. Blow by is a cause of acid in the oil. Basically oil oxidizes when temperatures get hot. When it oxidizes it breaks down into smaller molecules, some of these are acidic (oil is made up of fatty acids so while it is not acidic itself it is made up of acidic parts). When blow by happens those gasses are really hot, with available oxygen (along with the oxygen in the crankcase). This results in most engines (unless they have a severe oil loss issue) having a gradual build up in oil acid levels.

Yes adding oil will see acid levels drop for a couple of reasons, #1 dilution. However even with dilution you never get back to where you are with all new oil. #2 Modern oils have buffers in them, those along with the dilution result in a lowering of acid levels. Again though you eat up buffer capacity and you never get back to where you are with all new oil.

I'm not a huge fan of synthetic oils. A good mineral oil base will perform just as well as synthetic, and actually have a lower rate of oxidation. While it is true that Synthetics tend to do much better on wear tests, but their oxidation rates, and that they are more "leaky" than mineral oils are added negatives. That said I think these days if you compare a synthetic to a high quality mineral oil, the additive package is more important than the base stock. When synthetics first came out there weren't good additives for them, that's no longer true.