The above link from KEW engineering is a good explanation of viscosity, in particular the KV100 and KV40 and graph 4 near the bottom of the page. When you are starting in the winter at a cool temp, say 0 deg C and your KV for 5W oil is 80-100 cSt but at operating temps (say 200C) 20, 30, 40 and 50 weight are within 10 cSt of each other I'd say the KV at the warmup stage and your application of the noise pedal are the most important. The Viscosity Index Improvers in synthetics are apparently better than the ones of old. Fuel dilution in modern injected engines is way less of a factor than in the carbureted engines I grew up with - and the exhaust is so much less offensive thankfully.
In the US, the owner's manual must state the recommended oil for the engine based on the oil viscosity used to obtain the EPA mileage estimates. (see post number 5 below)
Oil Viscosity, CAFE, and your Taco Engine
My concern with the FA20 DIT engine in my vehicle is less the starting and ending viscosity of the oil but the Dexos 1 Gen 2 (or GF-6, SN+) rating for LSPI. Ideally we would have a -5w40 oil that maintained ~13 cSt viscosity from -10C to 200C. But the amount of VII's to do that and maintain an OCI of 10,000 miles just doesn't exist yet.
Actually what persuaded me to use 5w30 was reading on this forum that only North American vehicles use 0w-20. My impression was the same Forester sold in Asia, Europe or Australia say to use 5w-30 and I think even 10w40 in tropical climates. And also that Japanese Subaru site that showed the different types of oil Subaru uses and the conditions to use them in. The FB25 was not designed to only use 0w-20.
The questions I have are: what cSt assumption vs design life was used when the engine specs were being put together. (There's gotta be a number of limit curves defining that vs cost of warranty claims and public perception of longevity). Can I cheat the design life out of the curve by using A or B oil? Or is the fuel I use more important? Or is the average ambient temperature? Anyone work at Fuji Heavy Industries?