10W-50
The first number is what base stock was used in the formulation without the additives.
So, a 20W number will never attain the flow characteristics when cold as 10W.
The 40 or 50 number is what the oil behaves as when it gets hot. It is still as thin as the base number would be at that temp, but it lubricates and gives a film strength as a base stock of that thicker viscosity would.
None of this speaks toward the oil's resistance to breakdown from the heat experienced. At some threshold the, additives or viscosity modifiers break down and stop performing. Which means the base stock is providing the viscosity index. However, that base stock can also break down with heat, carbonize, and stop giving lubrication as needed.
It is not easy to find the temperature breakdown index of the chosen oil. However, a true synthetic oil has a much tighter control of the molecules used in it's construction with little impurities. Refined oil always has some impurities, sulfur, ash, etc. A true synthetic won't have those, yielding a uniform molecule size and a more predictable and higher temperature withstand before molecular breakdown, and none of the impurities to scape metal as the oil breaks down.
What I do. I follow what Honda recommended 10W-40 for all weather conditions. But, I do occasionally get into hot weather and traffic (low air flow) which will damage the oil. As a trade off between economy and performance, I'll use a synthetic blend oil so as to retain at least some portion of the oil that can maintain lubrication at those high temperatures. When the clutch starts to drag enough that shifting starts to get stiffer, that signals me that I must change the oil to restore full properties. I still try to maintain normal oil change intervals, but those intervals get shorter when I actually do encounter high engine heat.
If money was no object, I'd just use full synthetic PAO oil Which is not "refined to an approximation of synthetic standards" but a true synthetic, manufactured oil.
Of all my owner's manuals, only the 72 CB500 makes mention of 20W-50 oil. From 74 onward the recommendation is 10W-40.
I can only speculate why the change, as perhaps the oils got better, or they had more warranty claims with 20W50 oil use? I don't have Honda internal thinking on the matter.
I've been swayed by urban folklore to use 20W50 on some of my bikes. I can't say there was detriment or benefit from doing so.
But, use of an oil outside of recommendation is easily defined as experimentation. Since I don't care to take the engine apart any earlier than necessary, I'll stick with the viscosity recommendation of the manufacturer, and try to select an oil that can survive high heat. I'm aware of no certain detriment for doing so.
Cheers,
