Story about esters:
Driving 2005 Unimog U500 camper back from Mongolia in July 2016. Left front portal gearbox started leaking from inner seal just outboard of CV joint which is out in open. Vehicle had ~93K miles. We were camping about 250 miles NE from Moscow when I noticed it. I use Delvac Synthetic GL5 gear oil 75W90 as per MB recommendations.
I had 2.5L x2 unopened Penrite GL5 75W90 that I bought in Australia in 2010 for top-up when needed.
I contacted Unicat in Germany (they built camper) and they sent me directions to 1 of the 3 Unimog-only MB service places in Russian Federation, fortunately on Moscow outer ring road.
Penrite says their gear oil that I had inside truck is very high in esters. I stopped every ~20 miles to check level and refill if needed. It gradually stopped after ~80 miles and never leaked again!!
I did have seal replaced at Berendsen near Karlsruhe (another Unimog only MB dealer) over the winter.
But esters really do cause (beneficial) seal expansion, I am convinced.