I'm sorry Petunia but there really isn't such a thing as a low compression diesel. Even small Diesel engines run at over 200 psi compression. All diesels rely on high compression in order to ignite the fuel. If you have low compression good luck getting it to fire.High compression diesel ... or low compression diesel [ most small diesels ] ... same theory, depending on age any engine will have a minute gap in the compression rings and sealing of valves/rings may or may not be absolute perfect? Regardless of any variation, to wind an engine by hand is possible due to Einsteins Theory of R-Evolution, at low speed of revolution air/gas/or matter can escape through these minute gaps much faster than if a piston is at full warp speed.
All one needs do is try tightening a crank nut to the specifications of ''no one can achieve that without a rattle gun'' or even worse is trying to undo a crank nut from factory specs without a rattle gun.
It can leak past the piston rings and valve guides. No matter how good an engine is, there has to be some form of clearance on these items as they need clearance (no matter how minute) to be able to slide past each other. Piston rings also have expansion gaps in them which can allow oil past.No one has yet explained how oil gets in the cylinders? Everybody says it "might be" in there.