I would suggest that you don't need an oil "for diesels".
That's not good enough!
You have a turbocharged engine, and usually an engine manufacturer will require something more than just a "diesel" rating, in order to properly lubricate the turbine, which may be running at well more than three or four times the maximum rpm
of the engine. I don't know which of those oils are rated for use with turbocharged engines, or what Yanmar
specifics, but would strongly suggest looking into that aspect.
As to synthetic or not...there are arguments to be made both ways but unless there is a specific problem with synthetics and YOUR specific engine, they tend to be the result of better quality control and to have higher additive loads, as the premium product in any lineup. I'm a firm believer in synthetics, unless there's some problem (i.e. the engine consumes a lot of oil or the seals
are incompatible) with a specific engine. I've seen the way synthetics simply shrug off degradation and tarring from heat, and it impresses me.