Good question. I've come to the conclusion that often nobody knows the answer. The reasons might be:
1 There are too many variables (oil type, engine type, use conditions) and the test cycle is too long for anyone to do the experiments needed to find out about wear.
2 There's lots of swapping of products, both raw and finished, between products, ranges, brands and suppliers, and only insider knowledge can tell you about that, and the swapping may change over time.
3 Many 7 owners change oil much more often than the period manufacturers have to design it for, so success is defined differently for the two.
What happens in practice? We make decisions based on tests (where they do exist) that don't meet our uses, plausibility based on formulation rather than testing, and brand loyalty. And price.
What is logical behaviour under these conditions? (Captain.)
1 Be conservative: our own engines are not the best place to do experiments
2 Trust experts.
Jonathan
Edited by - Jonathan Kay on 19 Aug 2013 16:23:51