What's the possibility the baffle under the draft tube down inside the crankcase might be missing? That will allow oil thrown off the crankshaft during normal running the be thrown up inside the tube where it can get siphoned out by the airflow of highway driving forming the vacuum that it's supposed to at the end of the tube. Below 20-30 MPH there's not much vacuum, so the oil loss will only show up on a road trip, not around town.

The baffle is held into place by two machine screws (stove bolts) through the side of the block just below the draft tube hole. If those screws are missing, oil can leak out of the holes, also.
Jerry

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