Yanmar does sell their own brand of 15W-40, that's what the dealer gave me for my free 50 hour service change (I opted to do it myself). But for the 200 hour change I'll probably use Rotella.
Which rotella? Some versions have lots of high pressure point wear additives. Other 15-40 Rotella is low additive, low ash and requires roller cam followers etc.
Rotella's great oil. I always used Marathon MultiPower III 15W40 for heavy duty until they came out with synthetic. The backhoe has Pennzoil synthetic in it now,and my cars all have Castrol Syntec full synthetic. But I've always heard great things about Rotella.I just happened to be a Marathon and Sohio fuel truck driver/repairman back in the seventies and eighties during wintertimes. don-ohio ^)