I own the same model and mine doesn't burn oil. If your engine's inside
is clean and its done < 250K miles there shouldn't be excessive blowby.

Biodiesel 100 cleanses the fuel system and so clogs the fuel filters but
that's all it will do. Bio is NOT the problem. Bio 20 doesn't cleanse.

I suggest you check the vacuum pump (located on the front of the engine)
which exhausts the air from the car's vacuum system INTO the engine
(prior years had a tube to the intake manifold). Excessive blow-by
implies a vacuum leak, perhaps from a door lock, the climate control,
brake booster etc.. I suggest you test each of these vacuum systems'
ability to hold vacuum. Also look for an open vacuum line.

Inside the engine's valve cover is a baffle whose function is to
separate the lube oil from the crankcase grasses. The baffle may be
clogged and inoperative allowing oil to pass.

Overfilling the lube oil (level s/b between the dipstick's H & L notch)
will force oil UP the air cleaner's drain tube on hard acceleration. The
paper air filter gets soaked in that event making it difficult for the
motor to "breathe". Many shop are careless and overfill.

Sudden oil use could also be from a worn turbocharger seal but that
wouldn't create blowby.

The mileage decrease is, IMHO, from preignition of the lube oil and will
clear when the oil consumption is fixed.  Expect 24 - 25 mpg.

If nothing found from these suggestions a compression test will reveal
the motor's condition. Its compression spec. is 319 - 348 psi with 218
psi the minimum with 44 psi allowable difference between cylinders.