[mr0077]

Yeah, not really "leaner", just seems that way, as with the lower mean effective pressures at cruise, the combustion process takes longer to complete, so more total advance is needed for a complete burn during cruise...more complete burn results in higher effiency and more power for a given throttle position (during cruise), so less throttle opening is needed to maintain a given cruise speed...that equals better mileage, so "leaner" on a per-mile basis...
Oh, well...a well set-up vacuum advance will improve mileage, but not having a vacuum advance won't hurt the motor if you don't want to run one.