Try adding a couple of quarts of kerosene or Diesel fuel to the oil, enough to bring the oil level about 2" above the full mark. Then run the engine at a fast idle (1200-1500 RPM) for about 15 minutes, with no load on the engine. Drain the crankcase and change the filter while the engine is hot, and refill with straight HD-30 oil. It can't hurt, and you just might de-carbon the ring grooves enough to get a better seal against the cylinder walls.
Jerry
Your advice has always helped me. But I must say that this advice is "over my head". 
I poured kerosene all over this engine and sprayed it inside and out with kerosene when I cleaned it up. Sprayed kerosene down inside the cylinders and all over the head (still bolted on). Cleaned all the oil and carbon off the rocker area with kerosene when I removed the valve springs and rockers to put new valve stem seals on. Clean enough to eat off the engine. The only thing I didn't clean was where the head bolts to the block. Never removed the head.
So adding kerosene to the oil will wash out carbon deposits, right? What's all this talk about carbon on the ring grooves? What in blazes is that? 
