Paul, you can't judge the condition of rings by whether the piston moves from side to side. A compression test should show you whether the compression rings are leaking - they rarely do on Hondas, but if they did, blow-by could cause oil to be blown through the PCV system into the breather pipe. Eventually the oil rings wear out, but that causes blue smoke from the exhaust, not oil in the breather pipe.
If there is oil in the breather pipe, one possibility is that the mower has been tipped over, which fills the PCV compartment with oil, and often the breather pipe as well.
