And 20w-50 is alot closer to the 20W the manual calls for than 10w-40 is. Manufacturer's Rec....right?
I wouldn't assess it that way. Oil viscosity varies with temperature. Which means it flows slower when cold and the film barrier it provides between bearing surfaces get thinner and weaker as it warms.
All oils do this. Enter Multigrade oils, which have polymer additives that change viscosity properties as heat is added. Yes, it gets thinner, but it behaves and provides a film barrier equal to the higher weight grade on the label.
When the engine is cold 10W-40 behaves like 10W which flows easily about the engine, and provides all the film strength needed to properly lubricate engine components, even below freezing. As the engine warms the oil, it behaves like a thicker oil. When it gets to 180 degrees, t performs as a 40wt oil which is the thickest oil that Honda recommends for the SOHC4.
If you were to use a straight grade oil, my 77 CB750F owner's manual doesn't recommend more than a 30Wt oil in straight weight and assumes the engine will never be operated with outside temps below 59F using the grade oil.
There is NO factory recommendation/requirement for 50W oil at any temperature.
If you wish to ignore factory recommendations and insist that more is better, why would you stop at 50 Wt? Why not go to 70 or 90 wt?
Isn't having $90 better than having only $40 or $50?
On the one hand if you never travel to anyplace and start the engine cold where temps are 32F or below, then technically you are within bounds using an oil that can't behave at less than 20W viscosity.
What is of concern though, is not so much how it behaves while at operating temp. But, how it behaves and functions as it warms up. So, how you are using it matters. Short runs, may never warm the oil much, leaving the engine with marginal lubrication using 20wt. 10Wt flows and behaves much better in this regime. However, if you start once a day and ride for hours, cold performance is less critical, because the operational time spent while the oil is cold is a much smaller percentage of total operation time on the engine.
The point is, that the manufacturer says you needn't worry about oil viscosity or tailor the grade for the specific treatment you are doling to the machine, by simply using 10W-40. It's all weather, all condition, street recommended. And, you don't need to be a guinea pig or use your engine as a test rig to determine the efficacy of an alternate oil selection.
Check out the vast difference in viscosity when the oil is cold, as in during start up.
I see this chart and say "Aha, THAT'S why Honda said to use 10W-40". I don't understand why so many assume Honda knew less than internet posters. On the flip side, if the machine is modified to produce more internal heat (max power), or is operated to spend more time in an environment of high heat, (Ie, outside normal/racing) then you are already in the experimentation arena, and have chosen to be a guinea pig.
Roll the dice and take your chance!
To me, stock engine = Honda recommendations. I've not seen a convincing argument for change other than statements surrounding the terms;"I like it".
I did try 20w-50 some years ago in my 550's. Clutch drag, gnashing of gears, clunk and lurch into gear (particularly when cold) was far more prominent. Cold operation was clearly harder on the trans and shifter mechanism. Didn't actually break anything. But, I'm sure there were increased wear factors with all that hammering. With 10w-40, start up and go was far less problematic and smoother, particularly on cold mornings.
Chart from:
http://widman.biz/English/Calculators/Graph.htmlGood luck on your choice!
