My first look would be to see if the breather hose or vent was clogged, take it off from the filter and blow into it, there should be little to no restriction, if it isn't real clear, then 99% of the time where it connects to the valve cover the screen
wire or whatever is used as an oil / air separator is clogged. Taking the dip stick out when it's running is also a good check, if there is pressure in the crankcase, you will know in a hurry. Just slide the stick up a little so you can out it back in a hurry, no need to take it all the way out.
Step two, take oil filler cap off with engine running, there will be a pulsation of air as the pistons move up and down of course, but there should be no real flow of air. If there is a lot of airflow, not just a pulsation, then that points to blow by, stuck or broken rings or glazed cylinder walls maybe.
Never forget the exhaust elbow
and or has already been mentioned the exhaust hose, the hose in particular can really fool you, it can look just fine, but the inside can collapse when running, uncommon, but it happens once in a blue moon.
Did it start running a little rough, or just a drop in RPM, but still smooth?
You may have "lost" an injector, lost
as in clogged and lost
that cylinder, the way to check that is to loosen each injector line at the injector one at a time, you should get a significant drop in RPM and it should get a little rough as you loosen a line. I put a rag around the line as a little fuel
will spray or leak out when loosened.
Next is a compression
test, but that requires equipment
your unlikely to have, so maybe hire a mechanic
who has it.
If I had to guess, your symptoms seem to point to a cracked piston, but it could be other things as well, hopefully something simple.
Check the oil level again now that it has had a chance to settle, often checking right after shut down it will show low as there is much that hasn't drained back into the pan yet.
Half a quart in fifteen minutes is a tremendous amount