I use 90wt and only 90wt. I have never had a single issue that you guys are speculating about, by the time the motor is warmed up from my easy run, the chaincase is too. I have no chain stretch issues, very little metal material in the oil and havent had any chain/gear issues to date.
We got an oil testing machine one time that measured how much load an oil could take before being wiped off and the parts going metal to metal. It provided some interesting information, but right on the machine it said 'Do not test 90 wt.' So, we promptly tested it, and you know what? That thing couldnt create enough force to see what it would take to wipe the oil off and parts to go metal to metal. 90wt is GOOD oil and withstands the abuse that the chaincase puts it through.
Take it or leave it, JMO
Jake