Do you think there might be ANY possibility that the engine is DESIGNED to run on an oil that has the characteristics of a 20w oil? And that BRP's engineers did a lot of testing and found that 40w oil 'degraded' to 20w after 15000 km, so they specified the 40w oil?
If 5w20 oil started as 5w20 and STAYED that way for the specified interval, yeah, we would all be running it, but my manual calls for 5w40, so that's what I use.
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It is interesting that the shear characteristics for both the 998 and the 1330 engines are very similar. Allowing for riding styles, loading and environment variables. I would say that they are nearly identical (other than the relatively high oil usage of the 998, which really does not affect this conversation).
It is also interesting that Rotax, the maker of both engines, recommended 20w/50 weight oil for the 998 in the Aprilia motorcycle. The same engine that the V-Twin Spyders use with combustion modifications for increased torque at the expense of some HP.
BRP did not have a 20w/50 weight oil at that time. But they did have a 10w/40. And that is what their manual recommended in the beginning.
Since then, BRP has consolidated their lubricant offerings and now has neither a 20w/50 or a 10w/40 weight oil. But they do have a 5w/40 weight. And this is now their recommend lubricant for both the 998 V-Twin and the 1330.
The service interval for the V-Twin is still 4,500 miles (all the way up until the last production year of 2016). This has not changed. Which is interesting because 4,500 miles is about where the oil really starts to show a steep decline in viscosity. Makes sense to change at this point.
So, why would the 1330, which is arguably at least as hard on oil as the 998, have a service interval over twice what is required for the 998. It is know that there was a lot of pressure on BRP to reduce service costs and extend service interval times for the new Spyder engine which came out in 2014.
We can speculate all day long as to the 'WHY'. And everyone is entitled to their opinion. But you have to ask yourself. Was the original recommended 10w/40 a better choice than the 20w/50 recommended by the company who manufactures the 998 V-Twin? And is the 5w/40, now recommended, a better choice than the previous 10w/40 recommendation?
I have to say, all very interesting. Just don't expect any universally satisfying answers.