Remember that viscosity is a floating target. There is no oil that maintains the same viscosity across all temperatures. As oil warms, it thins. Period. Viscosity modifiers are how we end up with multi-grade oils, such as 10W-40. The SAE 10 is the base oil, then modifiers are added, which, with heat, uncoil their chains and create friction in the fluid. This internal friction mimics the viscosity of a higher grade oil, which makes the oil behave like it's higher rated number.
Anyhow. Back to the beginning. Temp vs Viscosity. It's called kinematic viscosity and the temperature is always important. If anyone ever asks you "What's the viscosity of this oil?" your response without hesitation should always be, "What's the temperature?" Look at manufacturer spec sheets and you'll see that it typically has multiple temps that it defines a viscosity for. For example, ISO grade oils always have viscosity defined for 40C and 100C, shown in centistokes cSt. Temp is just as important as the rated viscosity grade.
Its very possible (in fact likely) that there is some hot temperature where 20W-50 has the same viscosity as 10W-40 at the rated hot temp, as outlined by the SAE. By recommending 10W-40, Kawasaki has stated that our engine does best with a viscosity of X cSt at this temp. If you find some temperature where 20W-50 has the same viscosity as 10W-40 at the SAE operating temp, and then have data to show that your engine meets those temp demands, go for it. Other than that, I see no reason to mess with the hot grade for these engines. Remember that the highest temp inside an engine is in the jugs, where the rings slide across the cylinder walls. This temp is NOT highly affected by atmospheric temps during normal operation, but IS highly affected by air/fuel mixture, ignition timing, engine loading, etc. However, the cold temps ARE affected by atmospheric conditions, as you're starting your engine at whatever temp the air around it is. This brings up the first number of a multigrade, but that's an entirely different discussion.
To surmise this whole post: viscosity and temps are intimately related. To make a change to viscosity, you must have a well defined understanding of the temperatures that will be experienced by the lubricant. Engine RPM is important because it gives you an idea on how much oil flow there should be to transfer heat appropriately, but that's only a small portion of the decision. If you reaallllly want to change oils, you need to get the special logarithmic-scale graphing paper, plot the kinematic viscosity of each oil you're considering, and then draw a box with the outer limits of your temp range and outer limits of your viscosity. Whichever curve fits the biggest amount inside the box is the "best" choice. There's various little calculators to do this online, so do some research before randomly deciding to change engine oils.
Link to kinematic viscosity:
http://www.viscopedia.com/viscosity-...es/engine-oil/
FWIW, I really don't think the weather in SF Bay area is significantly different enough to justify straying from the "normal" 10W-40 or 5W-40, but this is just a SWAG without any data to back it. AKA, anecdotal. But do the research and prove me wrong. You'll learn something along the way and your engine will get the best lubricant for it, regardless of who is right and who is wrong.
