The second number, "40" indicates how viscous the oil is at operating temp so as far as weight of oil is concerned, anythingW-40 should all be the same viscosity when you are shifting after the motor warms up. 40 weight oil is more viscous than 30 weight oil. 30 flows better but 40 must stick to parts better. What makes shifting smoother and more definite, sticking or flowing? I can't answer that. I only have used 40 weight oils but the Gen1 service manual specifies 30 weight oil for temps up to 86°F, 40 for up to 104°F and 50 weight oils for over 104°F. If you run the bike when temps fall between 86 and 104, you could try each of those. If your engine gets really hot, 50 might be the best.
As the oil degrades from use, there is more of a tendency to miss a shift but I would notice it first by the shifts between 1st and 2nd sticking in Neutral instead of passing right over Neutral as it should. This is not a big problem since 1st and 2nd are usually a pretty low mph shifts but it can give a guy a scare to shift up or down between 1st and 2nd and end up coasting in Neutral around a corner instead of on the throttle. Conventional oil and for me, Rotella T6 synthetic seem to wear out shortly after 2500 miles. I use Rotella T6 synthetic and change at 2500 so I don't usually run into any shifting trouble from worn out oil. It would seem likely that worn out oil would induce miss shifts under harsher loads if it does that in easy shifts between 1st and 2nd.
In my opinion, fresh oil is going to make the biggest difference in protecting against missing shifts. The exact weight is probably only going to produce a noticeable difference if you are doing extreme shifting often enough to be aware of what is better and what is worse. Chrly would probably have the answer to what the best weight for smooth shifts is. I wouldn't want to sacrifice flowability for better shifting. There's a lot more that needs lubrication than just the gearbox and you might not be able to feel how smoothly those other parts move.
* Last updated by: Rook on 9/21/2020 @ 4:15 PM *
08 MIDNIGHT SAPPHIRE BLUE ZX-14 Now Deceased, will be resurected
2024 ZX-14R bran friggin NEW!