It would definitely be helpful to know where the oil is going. Oil leaks are usually evidenced by some sort of oily, grime accumulating under the engine, transmission or frame.
That said, there is a difference between tolerance and clearance. The Machinery Lubrication author confuses things by using these terms interchangeably. I believe him when he implies that "clearances" haven't changed much over the years.
Tolerances, however, are a different matter. Tolerance means how close the part (piston, cylinder, etc.) should be machined to meet the design criteria. Years ago, plus or minus 0.005" might have been an acceptable tolerance. These days, I suspect that tolerances are tighter because modern mass production equipment is more capable of actually meeting the tighter tolerances with much less dimensional variation.
