Vladimir Zakharov Oct 2, 2015:

Seepage vs leakage I kindly ask English native speakers to comment, but my understanding is:
- Leakage means some, well, flow — however slow
- Seepage means formation of drops, not something actually flowing.
As far as truck manuals are concerned, I think the two are more or less interchangeable.

From Oxford dictionaries:

seep·age
ˈsēpij/Submit
noun
noun: seepage
the slow escape of a liquid or gas through porous material or small holes.

leak·age
ˈlēkij/Submit
noun
noun: leakage; plural noun: leakages
the accidental admission or escape of a fluid or gas through a hole or crack.