Honda obviously spec the most suitable grade for a wide range of expected conditions, so a 10W30 is fine for the majority of owners.

However, depending on where you are in Spain, or if you are putting the motor (transmission) under additional load/stress, on track for example, a 10W40 may be beneficial if you consistently experience very hot ambient temperatures.

A synthetic oil is widely considered to be better than a mineral oil, however, for use within the NSR transmission under general riding conditions, it is likely to make very little difference. The oil should be changed every 2 years, and a mineral oil should be quite suitable for this duration.

10W30 and 10W40 are the same viscosity when cold, but the 40 is obviously designed to operate at higher temperature. You may find that the transmission is more "clunky" or notchy with the 10W40 for longer. (Many people experience a slicker gear change when the motor [oil] is hot, which would mean the 10W30 gives a nicer feeling, sooner.)

The short answer... 10W40 synthetic won't do your transmission any harm at all, but you may notice harsher gear changes.
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Andy.
NSR-WORLD.COM

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