I don't know why Yanmar would sanction CH-4 rated oil.. Most diesel oils today exceed that rating. (CI-4, CI-4+) There were some detroit Diesel motors that couldn't use normal diesel oils but it was because they used a lot of silver in their bearings. Yanmar does not. Mack Boring, as was suggested, is a great place to talk to about this, but I would think Rotella T would be a great oil for that motor!. I use Rotella T Synthetic in my 3 GMF.. Best check it out with MACK.. Please post the results of your inquiry to MACK for the record.. THANKS
The Yanmar website has this to say:
Which lube oil should I use for my engine and gearbox?
Any well known brand name oil for diesel engines will do, use mineral 15W/40 CF oil. Please remark if you have a turbo engine; make sure the oil is for turbo diesel engines. On gearboxes, it depends on what model you have, Kanzaki, ZF Hurth, Twin Disc, Newage or something else.
This from an SAE paper ..
Abstract:
This paper reviews ASTM's work in developing the new API CH-4 diesel engine oil category for 1998. In focuses, in particular, on three new engine tests ~ Cummins M-11, Mack T-9, and Caterpillar 1P ~ which are juxtaposed on existing engine and bench tests in the API CG-4/CF-4 categories. These new tests ensure increased engine durability, while operating at high temperatures and high levels of soot in the crankcase. The high soot levels expected in 1998 emission-controlled engines are a result of retarded fuel injection timing used to lower NO\dx, combined with high top-ring piston locations used to minimize particulate.
API CH-4 is the most robust API diesel engine oil category ever developed. It improves the quality of diesel engine oils for both existing and new engines, using both low and normal fuel sulfur levels. In addition, it will allow a more flexible approach to oil drain intervals, in accordance with the recommendations of the individual engine manufacturers
From another API (American Petroleum Institute) Paper:
API CI-4 oils are pivotal components in maintaining diesel engine durability using cooled EGR. API CI-4 is a higher quality oil than the previous diesel oil categories, so it will provide engine protection for both existing and new EGR engines.