Your terminology, as is so often the case with Americans, is faulty and ill-informed. British machinery does not "leak" oil.
The evidence of petroleum-based products you perhaps espy on exterior casings and perhaps in the immediate environs of the machinery, is the key to a dynamic anti-corrosive system designed to prevent harmful oxidation of the exquisite British castings in salty environments, such as the US in Winter, or at ocean resort communities elsewhere in the Empire.
The anti-corrosive agents, which also may, or may not, also have some slight lubricating or cooling effect on internal rotating components, are contained in "sumps," * which have scientifically calibrated orifices designed to release them in a timely, efficacious, and carefully metered manner exactly where needed.
What is doubtless confusing to you is that these metering devices may, or may not resemble what you American roughnecks ignorantly call "seals," or "gaskets."
*PLEASE VERIFY THAT THESE "SUMPS" ARE FILLED WITH THE PROPER PROPRIETARY LIQUIDS EVERY 10 MILES, OR HALF-HOUR, WHICHEVER COMES FIRST. CORROSION PREVENTION IS EVERYONE'S JOB. Consult your owner's manual. Heading: Timely Maintenance. Failure to do so will most certainly void the extensive warranties that cover British machinery, some lasting several weeks.
After four Jaguars, a big Healy and three Limey bikes, I'm glad someone finally straightened me out on this issue.