This maybe a daft question. Have you had mower tipped on its side for cleaning etc and inadvertently tipped it air clear side down? This can let engine oil spill from crankcase via breather pipe into air cleaner. If this is the case just clean out oil pop in a new filter check oil level and you should be away again. the alternative could be wear in the engine causing crankcase pressure as you suspect Hope it is only as my first thought.
Smudge
February 2014
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