I also have the Yanmar diesel 3TNE74(C). My Yanmar manual for TNE series engines states: "Use Class CC or CD (API Engine Service Classification) oil". 10W30 is recommended for -5º to 87º Farenheit ambient temperature range and 20W40 for 12º to 105º F ambient temperature. I've been using 10W30 wt. Mystik JT-8 by CITGO. It is API Service CG-4, CF-4, CF/SH and is recommended for all diesel engines.