i have a X728 tractor with 690 hrs. It is used to mow 4 acres of lawn weekly. I bought it new in 2003 and it has served me well. Reciently it has started using oil(no smoke). It uses about half a guard per 8 hrs of mowing. Is this normal for a tractor with this many hrs ? I would like to keep this tractor for a few more years. Does anyone have a suggestion on what I should do ?