10W-60? How often do you change your oil? With the oil also lubricating the transmission the high molecular weight polymer coils added to improve viscosity are being shredded at much higher rate than in applications where transmission is not lubricated by the same oil as the engine. The bigger spread between the winter rating and summer rating the more polymer additives is needed. The polymer coils are bunched up in cold and unwind into linear shape in hot conditions to keep the viscosity at an acceptable rate and not allow the oil to become too thin. Low spread viscosity oils do not need or have a lot less of the polymer coils. The polymer coils start to shear as soon as you start the engine and the WS spread start to decrease. The rate of shearing if the oil also lubricate tranny is significantly higher.
So in effect when you pour the oil in, it's 10W-60, but as soon as you start riding the higher viscosity number starts to rapidly decrease in application where the gears in the tranny, that is being lubricated by the same oil as engine, start to shear the polymer coils. Why do you need 10W rating that is rated down to 0F when you can use 20W rated down to 32F? I'm sure it's a lot warmer than 32F when you ride so the viscosity of the oil is sufficient to lubricate on start up.
Don't take this the wrong way, just to something to think about.
i won't take it the wrong way, but when you do this for a living the difference between desirable theory and practical reality has far more influence on what is used. points i'd make, in a rambling fashion.
two of the manufacturers i deal with - guzzi and mv - recommend the 10w60. guzzi does not have the issue of a gearbox chopping up the polymer additives. the mv does. in reality, most guzzi owners balk at the cost of the 10w60 anyway, and prefer the 3100 15w50.
in the same engine, would you say that it's not worth running a 10w60 as opposed to a 10w40 due to the extra damage to the 60's polymer chains? how long does it take for their additives to be effectively the same? do they deteriorate at the same rate? most importantly, what damage would you expect to see as a result?
20 cold weight does impact starting of some bikes here in winter. as such, i'd not use a cold weight heavier that 15, regardless of the theoretical recommendations. if a customer thinks there's a problem, regardless of the theory involved, there's a problem. often fixed at my cost because i should have known. i don't like that bit.
we used to use 10w40 ultra 4 in air cooled bikes that were noticeably noisier when hot that using the 15w50. so i won't use 40 as a hot weight.
motul make different/non expected weights in some oils. 7100 is 10w40, 10w50, 10w60 or 20w50. all the same price, so i use the 10w60. if they made a 15w50 i'd have a drum of that and a smaller amount of the 10w60.
i could recommend to people that they change oil twice a year to allow for temp variations and/or the theoretical considerations of the long term use of wide viscosity range oils (i understand that bit), but i'd be the only person in town doing that so i'd be looked upon as an arsehole trying to get more money out of customers. and lots of these owners are flat out seeing the reason to change oil even once a year. most of the bikes i work on are time based, not km in terms of servicing. and some will prefer to (well, will only) see me every two years.
some owners will do whatever i say. most think about the money and inconvenience side of it first. getting the bike to me for the work is often more of a hassle than anything else.
some of the 2v air cooled engines that are commuters will see quite wide temp variations, even in winter, over the course of one ride. in that instance, i see their oil requirements as far more extreme than a water cooled bike doing the same job. most owners wouldn't think their 620 commuter needs better oil than their 1098, but in reality it probably does.
if i could, i'd probably use penrite oil. locally made, they have some great stuff and the comparable top range oils are cheaper than motul. but some customers would scream and demand i drain it out (and probably give it a full engine rebuild) if i did that. it has little to do with actual reality, just perception and mob mentality. i actually had that happen when i put 7100 in a bike that had used ultra 4 all its life. owner was furious. the fact you couldn't, at the time, locally buy the 15w50 ultra 4 was a moot point on him.