The general input from corporate Yanmar is to use a 30 Micron filter in the Primary Filter as depending on certain models 20 or finer can lead to fuel starvation issues as the fuel is being sucked from your tanks by (depending on the model) a mechanical fuel lift pump, the interesting thing is that for conventional engine models (non-electronic) they show generally to use 10 micron as the engine mounted filter, the 2 micron is the recommended filter for there electronically controlled engines,
That said with if using the 2 micron it's recommended to use the 30 micron as the primary unit.
The bottom line is really check with your operation manual as it will be specific as to your requirements for that engine ONLY,
Cheers Steve