When the engine is cold (glow plug in and seated), reach under the chassis and try turning the flywheel a complete rotation with one finger. If it will turn completely through the rotation with little effort, your piston/sleeve has lost its pinch and is in need of replacement. If it still gives quite a bit of resistance, you're probably looking at a different problem.

My guess, with 8 gallons on it...is that it's time for a rebuild. That's a "general" life expectancy for a big block that's been tuned right and taken care of.

If you do opt for a rebuild, check for slop in the bushings of the conrod. Take into account not only the price of a new piston/sleeve, but the conrod as well. If you have to replace all three, sometimes it's just a little more $$ for a entirely new engine.