So forgive me if I'm out to lunch here, but isn't the difference between 5W40 and 10W 40 only relevant at startup. I was under the impression that as the oils (these two in question anyways) come up to the standard operating temperature their are no differences in them?
I put Amsoil 10W40 in both our bikes and pretty much forget about it until it's time to be changed. Too much oil talk gives me a headahce.........unless we're talking extra virgin olive oil drizzled over fresh cut tomatoe's with some basil and cracked sea salt on them.....
I can only share my opinion from a guy that used to sell oil, and has been a rider and mechanic most of his life, and who has a father thats also a mechanic ( automotive) and who has many friends, and relatives that wrench for a living.
A thinner oil sounds good on paper, but unless the temps are too cold for say a 15w40 oil to flow okay at initial start, the 15w40 oil protects better.
It just seems to leave a better oil layer on bearings etc. to protect them over time.
Also bikes that have the oil shared by the motor, clutch and tranny has a different need than a car or pickup.
the extra gears etc. break the oil down quicker, causing it to get thinner and thinner as you ride.
And the larger spread of say a 5w40 as compared to a 15w40 makes it have additives in greater amounts, that are suceptable to viscosity breakdown.
Almost every bike I have seen that has suffered say cam shaft bearing failure was due to the owner running a thin oil.
After rebuilding their engine, i tell them to only use say 15w40 or 20w50, and change it regularly....the exception being if they ride in temps below freezing, and the bike is not always started in say a heated garage.
I just fixed a KLR650 actually that suffered severe bearing damage due mostly I suspect..and I say I suspect as I do not have iron clad proof, just an educated guess.......from the owner using 5w30 oil in it.
it had low mileage, and was severely damaged.
the bike was a 2006 KLR 650 with just 17,000 kms on it.
I tore down my old KLR650 engine with 62,000 kms on it, and the engine was fine.
It was ran on nothing but rotella 15w40 from about the 100 km mark until i tore it apart to look at it, and then make it a 685 cc engine.