A new engine doesn't have the advantage of a 'perfect' seal. Liquids are incompressible, so by adding the oil, and slightly subtracting the oil's drainage through the piston ring gaps, you have 'perfect' compression.
In other words, take a new engine with the piston rings left out, use a thick enough shot of oil per cylinder, and you get 300psi.
Your shots of oil just make up for the wear.
Having said all that, I'd bet that new 2.5 engines fell short of 300psi.
Perhaps look for other causes for positive pressure? Worn valve guides/seals?