I do my oil changes at about 6000 km's (3800 miles) with Rotella T6 5W40 (It's JASO MA certified and synthetic whereas the stock GN4 oil is 10W30 and I believe is dino/conventional oil).Thanks Motorboy. I look at the oil in the sight glass, and when it starts getting dark, it's time to change it and I expect that long before 8000 miles. It's getting brown already! And to confirm, if I change only the front sprocket to a 15T the max torque will be reached around 7500 rpm which will let me cruise at 65mph without stressing the engine?
Sprocket change doesn't do anything to the torque or HP, nor where your bike reaches it's max torque/HP. What it does is change how many revolutions you need to do in order to drive the wheel the same distance. If you understand bicycle gearing then it's immediately translatable.
But basically, just because you change the size of your sprocket doesn't mean that your legs can output any more or less power. What it does do is let your legs spin the cranks at a lower RPM when going the same speed.
Basically, with the 15t sprocket highways will be less "buzzy" because you're in a lower RPM for the same given speed. I personally think that it also makes the gearing better and more usable especially starting off at a stop light and when at high speeds on the highway (I do a buck twenty at 7000 RPM with the 15t whereas before would need to be sitting at 7700RPM to do the same speed).
I did the calculations once before and IIRC the numbers I got were 100 kph at 6500rpm (~60 mph) on stock gearing, and with the 15t sprocket you're able to do 110 kph (~67 mph) at the same 6500 rpm. You do gain top speed, but you don't actually "lose torque" because torque doesn't necessarily equal acceleration. On two sides of the extremes; Imagine an ocean freight ship that has tons of torque available but doesn't actually get up to speed, then on the other side imagine the nimblest of sportbikes that doesn't have as much torque available but can accelerate fast.
Also note that you can't take this to the extremes because you still need to consider power available to you from the engine. ie. don't try putting a "Dinner plate" sized front sprocket (ignoring space limitations like an 18 tooth (+4)) and a "shot glass" sized rear sprocket (eg. 32 tooth rear (-4)) and think that with this combo you can achieve 200 kph if you had a long straightaway because the engine will not be able to produce enough power to overcome the parasitic drag (which is exponential) of moving through air (which is technically considered a fluid.