There are a number of places oil can leak.
Losing that much oil sounds like like it is certainly an oil leak and unlikely due to blow-by.
If you are not confident in diagnosing and replacing seals yourself you will need a mechanic to do this.
Look under and around the engine for places that look gunked up/really dirty, this will be the area the oil is leaking.
Leaks can typically come from the following places but is not
limited to them.
- Rear Main Seal (Big seal on engine crankshaft on
the side facing the transmission).
- Front Main Seal (Opposite side of
- Camshaft seals (Smaller seals that seal against the
camshafts at the front of the engine)
- Valve Cover Gasket (Large
sealing gasket used to seal the valve cover to the cylinder head)
- Oil pan gasket (Large seal that seals the Oil pan to the Engine Block)
There could also be a leaking oil line or etc if it has any external oil feeds (for turbos, or for Variable Valve systems).
There could also be a crack in an oil galley.