"A little castor oil" actually increases compression over a "dry" cylinder. (Old tractor tick for disguising a needed ring job - pull the plug and squirt Marvel Mystery Oil in the cylinder).

Anyway, pull the glow plug and turn it over, then put it back in and compare the two. It should be harder to turn over with the plug in. Also, try doing it without the leverage of the prop. Seems tighter then, henna?