Have a friend in the club who has a DEERFOOT 62. Has completed an around the world cruise over 3 years plus a Med cruise. He as a big turbo'd Yanmar diesel that has 6000+ hours on it. He uses 15-40 Mobil 1 synthetic oil. Any one have a reason why a synthetic oil can or can not be used? I normally use Delo 400. Don Lawson #1005