My Series 3 dripped or leaked oil like a sieve until I stripped and totally rebuilt the engine and gearboxes and from then on, not a drop, then I sold it and apparently 22 years of cossetted life later I am assured it is still oil tight.
My 1995 300TDi Defender 90 also dripped like a good un, until I also took the engine and gearboxes out and totally stripped them down, even though they had only done 30k miles, from then for my next 6 years of ownership 'Blue Louis' was as good as gold and didn't leak a drop.
From my experience and the shared experience of many LR owning friends, if you put them together correctly using high quality gaskets and sealants then in most cases they will remain oil tight.
Phil