When you check/change the gear oil (90 weight), and have it on the tractor, carefully lift it as high as it will go, block the rear end of the mower up with suitable stuff and look at the blades/bottom of gearbox, to make sure the gearbox hasn't blown a seal down there (Note: if the gearbox has gear oil in it when you check it, the seal is probably OK). Make sure the blades aren't bent and are about the same for both (should be balanced). If all is OK, you did well... BobG in VA