Got a Kawi 31HP (Super Z 72" side discharge) with 150hrs and it doesn't use oil. I changed the oil at 11hrs then every 50hrs after that. Changed the outer air filter once so far. With the outer and inner air filter I find it hard that dirt made it's way through. The inner still looks new.
When you say dirt gotten in around the intake is he thinking the intake gasket? Could the engine got hot and warped something? Fins cleaned? I would imagine that the rear discharge decks could kick more dirt/grass to the engine area.