have a 23 hp mitsubishi diesel w/ 1800 hrs that we have ran 10w 30 in since new. 10w-30 is what the owners manual said use. CD & SF are the oil spec requirements listed under the hood. We have been running regular (not synthetic) castrol 10w-30. We have 2 Kubota engines and both of them call for 10w 40 and we use rotella. I know yanmars call for 10w 40 also as we recently had a pair of them in a boat. Just wondering if we should switch to 10w40 in the mitsubishi. I do not think it would hurt but kind of hard to change now after putting 1800 hrs on this engine and it still runs as strong as the first day we bought it. Thanks for any input.