If it's running OK now I wouldn't worry.
I'm pretty sure the filter on my '76 4/4 did not remain full. Don't think it ever appeared to be full, either. Check the casing on the plastic filter though, mine went soft after a few years and I decided to replace every couple of years after that - at under 5k miles p.a. it never clogged up.
Fuel certainly drained back to the tank after a couple of days - but the extra cranking (mechanical pump) for a cold start meant a little more oil circulation before the engine fired.
My first LR was a '66 IIa with a low compression petrol as far as we could ascertain - pulled better than the 2.3 diesel though
