I own a 1993 Mercury Force 40 horse motor. It is all original and I bought it two years ago. My question is about the metal shavings that I find in the lower unit when I change it every year. They have always been in there and the motor has ran the same way since I got it. When I put the boat in gear you can hear it but the motor never slips or makes noise once you are moving. Is this normal or do I have something else going bad.