No blow-by means the rings are good, as a compression test will tell you, or just turn the engine over by hand feeling a strong resistance from each cyl as an "air spring". Sounds like it isn't leaking oil and you find oil on the plugs, so probably leaking past the stem seals. If never done, it certainly needs new rubber umbrella seals on each valve, which often cures that problem. You must remove the valve cover anyway to do a valve adjustment (maybe every 40,000 miles). Just don't drop a valve down a cylinder - before removing the 2 springs insure that piston is at TDC (wire in spark plug hole) and secure w/ a magnet.