Since you already stated that there is no leaks shown....Check to see if your tailpipe shows the burning of the oil with smoke and residue. Check to make sure your PCV line is clear. Check to see if the air filter is clean.
Then do a compression check. Simple enough to do. You could probably rent the tester from an auto parts store that rents tools.
If the PCV line is clogged, it needs to be clear. It takes the air outside the compression cylinder back into the engine for compression. If this is clogged, there is pressure being built up behind the compression cylider. The air behind the piston on the crank, rods, camshaft, etc.
If there is oil in the air cleaner, you got blow-by, meaning the compression is bad and the compression pressure leaks past the rings and leads into the air cleaner.
Compression check. The numbers between cylinders should be in the 100-120's maybe even more depending on mileage/age, but the cylinders should be 10% within each other for good even compression.
And not being too stupid here, did you check the oil properly and add oil properly. You know of the joke of the blond about the oil?
