There's definitely a break-in period. If I had to make a wild guess, I'd say that by 10K miles, an engine should be running optimally, and how long that condition is maintained depends on the engine design, parts quality, driving style/conditions, maintenance and fluid selection, etc...<P>My B23F is stronger than ever at 243K miles (partly due to mods, but hey - it doesn't burn any oil!) and will hopefully stay that way for several thousand more before the inevitable rebuild is required. <P>