The answer to derek's question is in ET43's response. The oil never gets to a reasonable working temperature, so never evaporates off (for example) water vapour.
Without giving way on the question of lagging, how to, I balanced the carbs on another vee-twin, a Guzzi Le Mans II. It didn't take long to find out that balancing the carbs was not just a matter of having the slides at the same height. I used a Carbtune, a pair of manometers, initially mercury, later stainless steel (gauges are useless, not sensitive enough). Starting with the slides at the same height so that it idled evenly, I had to adjust mixture so that the pots remained in balance as the slides lifted.
When I'd done that, which involved a fair amount of changing idling settins, the motor was significantly smoother. Balanced, one might almost have said..... Same was true of a Ducati 900 Monster: the slides were at the same height, but a ducati guru balanced the pick-up for me and it was like a new bike.