I usually trade something in when the oil samples start showing higher amounts of metal. I have a Cat 980C with 15,000 hours, use it just for snow now. I think trading something in at 6,000 hours depends on what type of work you do with it, and if the machine is used daily. If I was doing demo day in and day out I would try and trade the machines in at 6,000 hours just because of the beating the pumps, undercarriage and motor take. Not to mention the stick and boom. My Cat 320BL blew the motor at 8,000 hours and it was mainly due to my own fault. It's $13,000 for the motor and $2,000 to put it in. I'll do that run it another year and trade it in for a John Deere 240.