I'm not an engineer, but I do have to ask myself how much of an engineering decision goes into motorcycle manufacturer's oil change interval.
How do they arrive at the 4000 mile number, knowing that an owner is liable to use any number of different brands of oil that still meet Yamaha's specs but have their own recommended oil change interval? If it was truly an engineering based decision, Yamaha would likely mandate that a specific brand of oil be used because that's what they based their tests on, and that 4000 mile interval was the result of actual testing. Or they would test several brands and mandate that those brands be used. But some oils, especially the synthetics, have change intervals that are markedly in excess of 4000; Amsoil's intervals start at almost 3 times Yamaha's recommendation. Does it make sense to follow Yamaha's recommendation if the oil manufacturer says their oil can be changed much less frequently? After all, it's a lot more likely that an oil manufacturer is conducting tests on their own specific product than Yamaha is testing every oil on the market that's JASO certified and meets Yamaha's specs.
Is that 4000 mile interval based on any probative testing, or is just a reasonable but arbitrary number?