I've switched 100k mile motors over to synthetic without any bad consequences.
If you read tests on the true tested viscosity of xxW-30 vs xxW-40 at vehicle operating temperatures, you will see there is virtually no difference. The difference is at startup. Don't forget that the owners manuals were written many years before the new oils were even on the market. If you want to know as much opinions about oil as is possible, go spend a few weeks reading forums on
www.bobistheoilguy.com.
What I think is funny is how opinions vary with some people trying to flush out perceived gunk with thinners and transmission fluid, MMO, whatever, and others wanting to run super thick oil.
The main reason I swiched to synthetic was to increase time between oil changes, because my collection of old drained oil was getting to be a mess. Yes, I know I can take it in to the oil change stations.