Diesel oil
I would not run regular gasoline engine oil in a diesel engines. Diesels run at a higher compression and need oils designed for it. I don't now if the Shell Rotell is diesel rated but if not I would drain the motor and replace the oil. I buy Chevron Delo 400 SAE 15-40 at Wal-mart (blue bottle). It is a diesel rated oil. (The "C" in the Cx rating indicates compression engine, I believe.) Some say you should use only a single grade oil (per Yanmar manual), but the grade you need depends on the climate (higher it hot climes and lower in cold), and I am with the group that believes that a multigrade offers the best overall protection for your motor. Blue smoke does indicate buring oil. This is pushing the limit of my knowledge, but I believe that usually it is because oil is blowing thru the sides of the pistons and into the combustion chamber. If the appearance of blue smoke was sudden as you indicate, it may indicate that a piston ring stuck, allowing oil to pass thru. I would limit use of the engine until the problem is resolved. If you don't have a manual, I would buy both the Yanmar manual and the Sealoc Yanmar manual (available on bookseller websites). Both are good resources that will help you understand the possible causes of your problem.
