I re-read an SAE publicatiob, 981445 where they were investigating the optimal time to change oil in a 1.6 L engine. It appeared that the oil was 15W-50 Mobil 1. (Turned out to be 11,000 KM) They factored n fuel costs as well as oil and maintenance costs. But one of the things I noticed is that in 14, 000 KM the viscosity was exactly where it started at. It dipped slightly after 5000 KM due to volatility but then slight thickening balanced it for the rest of the way. At 21, 500 KM it was a little over 2 cSts over grade. Pretty good. This article was publshed in 1998. Also TBN dropped from 8.32 to 5.37 which is respectable.
