I was thinking same that you must be doing oil top-up while returning, so your mystery is resolved yourself that in isb oil level is reduced by quarter and when you come back lhr, its back to normal level
regarding losing oil, I beleive there is no leakage as car is losing oil only on long-routes, still get a check if there is any clue of minor leakage.
incase of no leakage; car is burning some oil on long-routes, its a sign of engine getting little old or not well taken care off.
Nothing much to worry, switching oil brand might work, though it will not fix damage done to the block, if any. One thing, is there any smoke noticed?
Never used Petromin but heard alot good about that, you can try that