My radar is always up on this issue. My knowledge of oils, and break-in oils, is far too old-school. But, one factor I'm keeping in mind with all these new oil publications is that they're mostly oriented toward liquid-cooled engines, where typical oil temps rarely go over 220Â°F. Several members here, using thermometer dipsticks in hot summer, have reported oil temps in their air-cooled XS650s going up to the 280Â°F range. Please excuse this over-simplified approach, there's a mountain of data on this. At this higher temperature, the kinematic viscosity of the oil approaches that of Jim Beam whisky, and the oil's life expectancy (for non-synthetics) is severely reduced. What I'm concerned about is the oil's hydrodynamic cushioning properties, to control clatter. If you study this chart for a moment, see where the equivalent viscosities occur for various grades and temperatures. Note that the oil's kinematic viscosity in a liquid-cooled engine is in the range of 8-20+, whereas it drops to the 3-8 zone around 280Â°F.