Dennis, Mercedes used to claim that burning a quart of oil every thousand miles (20 hours of highway operation in a car) was normal, because they used an oil film and loose rings to prevent cylinder wall wear. Well...that concept got accepted here in the US, where auto engines routinely run 15-20,000 miles without burning more than an ounce or two. (400+ hours)
I don't know what "Yanmar" you spoke to but I would call:
Yanmar America Corporation
951 Corporate Grove Drive
Buffalo Grove, IL 60089-4508
And follow up the phone call with a fax or letter confirming the conversation. If they tell you this oil consumption is normal...I'd be shocked.
Which still leaves the question, as a second owner do you have any warranty rights? Or if this is just a service issue, where are you located? Perhaps someone can recommend a good mechanic if you don't want to do the diagnostics/repairs yourself.