Oil:
In the UK the outside ambient temperature doesn't get much below -5c in the winter and I don't even use my bike unless the Temperature is above freezing. We are, in this case, lucky to have the gulf stream. However In Canada, which parts of that country are in the same latitude as the UK, the ambient temperature can be -20c. A 5w40 oil (5 Winter 40 viscosity) will be still thin at upto -25 alowing an engine to firstly crank and secondly for the oil to reach vital parts quickly. Were as a 10W40 maybe too thick for cranking at -20c or take too long to reach vital parts. At the time of writing the ambient UK temperature is at about 10c and there is little or no difference between a 5W40 or a 10w40 or even a 15w40. Some reasons a 5w40 is more expensive is because the viscosity spread between 5 and 40 is so great only Synthetic oils can achieve this. 5w40 is also more suitable to shear effect.
Ambient temperature does not effect water cooled engines when hot as much as air cooled engines under normal riding conditions.
Rattles:
My bike does the same. A bit embarrassing when starting the bike with your mates present. I've read on various forums that Aprilia don't see it as a problem and it is something to do with the timing chain tensioner needing to fill with oil which works hydraulically. I don't know if this is fact but it is annoying all the same.
